The effectiveness of recruitment strategies on general practitioner’s survey response rates – a systematic review

Background Low survey response rates in general practice are common and lead to loss of power, selection bias, unexpected budgetary constraints and time delays in research projects. Methods Objective: To assess the effectiveness of recruitment strategies aimed at increasing survey response rates among GPs. Design: Systematic review. Search methods: MEDLINE (OVIDSP, 1948-2012), EMBASE (OVIDSP, 1980-2012), Evidence Based Medicine Reviews (OVIDSP, 2012) and references of included papers were searched. Major search terms included GPs, recruitment strategies, response rates, and randomised controlled trials (RCT). Selection criteria: Cluster RCTs, RCTs and factorial trial designs that evaluate recruitment strategies aimed at increasing GP survey response rates. Data collection and analysis: Abstracts identified by the search strategy were reviewed and relevant articles were retrieved. Each full-text publication was examined to determine whether it met the predetermined inclusion criteria. Data extraction and study quality was assessed by using predetermined checklists. Results Monetary and nonmonetary incentives were more effective than no incentive with monetary incentives having a slightly bigger effect than nonmonetary incentives. Large incentives were more effective than small incentives, as were upfront monetary incentives compared to promised monetary incentives. Postal surveys were more effective than telephone or email surveys. One study demonstrated that sequentially mixed mode (online survey followed by a paper survey with a reminder) was more effective than an online survey or the combination of an online and paper survey sent similtaneously in the first mail out. Pre-contact with a phonecall from a peer, personalised packages, sending mail on Friday, and using registered mail also increased response rates in single studies. Pre-contact by letter or postcard almost reached statistical signficance. Conclusions GP survey response rates may improve by using the following strategies: monetary and nonmonetary incentives, larger incentives, upfront monetary incentives, postal surveys, pre-contact with a phonecall from a peer, personalised packages, sending mail on Friday, and using registered mail. Mail pre-contact may also improve response rates and have low costs. Improved reporting and further trials, including sequential mixed mode trials and social media, are required to determine the effectiveness of recruitment strategies on GPs' response rates to surveys.


Background
GPs play an important role in the design and implementation of health services and health policy to improve the health outcomes of their patients [1]. Understanding and measuring GPs' knowledge, attitudes, behaviours, practices and their views on solutions to health care issues are paramount to improve the quality of health care. Surveys are a useful tool to measure a wide variety of issues that are relevant to general practice. It has been argued that GPs are often time poor, are difficult to recruit for research studies [1,2] and often have low survey response rates [2] whereas the latter has been contended by others [3]. There has been a steady increase in research studies examining methods to increase response rates amongst doctors [3]. Despite the greater evidence base for improving response rates, a recent review of the primary care literature between 2000 and 2009 showed that GP response rates to postal surveys over the past decade are relatively unchanged [3]. The authors found that the average response rate was 61% (95% confidence interval (95% CI): 59% to 63%) amongst 371 GP surveys. Reasons for not participating in research are many, including concerns about disrupting routine practice [1,4], time [1,2,4], relevance of the study topic [1,2], confidentiality [1,2] and receiving many surveys a week. The general practice workforce is also increasingly working part-time [5]. Low response rates can lead to methodological biases, such as non-response bias, time delays in research projects, budgetary problems [6], underpowered studies and raise an ethical dilemma in that people have been subjected to research that can not show its effect [7].
Several systematic reviews have examined strategies amongst the general population to increase response rates to postal, electronic and telephone surveys [8,9]. Whilst general population reviews [7,8,10] are useful, it is likely that some strategies are more effective in increasing GP survey response rates than others. For example, GPs are probably less sensitive to monetary incentives than the general population given their higher income status. Many of the strategies applied to the general population have also been developed and tested to improve GP survey response rates such as monetary incentives [11], questionnaire length [12] and pre-contact [13]. Two systematic reviews [2,14] have examined how best to increase response rates amongst physicians. The first systematic review included studies conducted until 1999 and found that pre-notification, personalised mailouts and nonmonetary incentives were not associated with improved physician response to surveys [14]. On the contrary, monetary incentives, stamps and shorter surveys led to increased response rates. The second systematic review included studies until 2006 and broadly explored two intervention categories: incentives and design-based approaches. The authors found that even small financial incentives improved physician response rates but that token nonmonetary incentives were less effective. Postal and telephone surveys were more effective than fax and online surveys, as were mixed mode surveys, first class stamps, shorter surveys, personalised letters, and studies endorsed by reputable professional organisations [2]. The aim of this study was to assess the effectiveness of recruitment strategies aimed at increasing response rates of GPs to complete surveys.

Methods
Criteria for considering studies for this review Types of studies Cluster randomized controlled trials, randomized controlled trials and factorial trial designs that aimed to improve survey response rates amongst GPs.

Population
The primary population included GPs or family physicians.

Types of methods
Any intervention that compares different recruitment methods of GPs to complete surveys. Interventions aimed directly at patients were excluded. Studies that evaluated retention strategies were excluded. Interventions that recruited GPs for clinical trials were also excluded.

Types of outcome measures
The primary outcome measure was the proportion of eligible GPs who responded to surveys.

Search methods
MEDLINE ( OVIDSP-1948 to December 2012), EMBASE (OVIDSP-1980 to December 2012), Evidence Based Medicine (EBM) Reviews (OvidSP -December 2012) and references of included papers, related literature and systematic reviews were searched. Major search terms included GPs, recruitment strategies, response rates and randomised controlled trials (RCT). The search strategy was built on previous searches [1,8,15,16]. The search strategy for Medline can be found in Additional file 1. Search results were merged into reference management software and duplicate records were removed.

Data extraction
Abstracts identified by the search strategy were reviewed and relevant articles were retrieved. Titles and abstracts were independently checked by SP. A random number generator was used to select about 10% of the initial MEDLINE electronic citations. SP and SWP independently assessed 106 citations. Agreement of 99% was achieved and a Prevalence and Bias Adjusted Kappa Kappa of 0.98 (95% CI 0.79 to 1.17) [17]. Full-text papers were screened to determine eligibility by both SP and SWP. Disagreements were resolved by discussion. SP and TV extracted data. Ambiguities were resolved by discussion.

Assessment of risk of bias in included studies
Two authors (SP and TV) independently assessed risk of bias using standard Cochrane criteria, including allocation concealment, random sequence generation, blinding of participants and personnel, complete outcome data and absence of reporting bias. Ambiguities were resolved by discussion. Intention-To-Treat analysis includes everyone who has been randomised to a treatment group, irrespective of participant's non-compliance (eg not taking part in the treatment), protocol deviation or withdrawal from the study. Treated analysis is considered as high risk of complete outcome data bias, therefore the results of the assessment shows this element as a separate item for additional clarity. We considered absences of other bias in the form of whether differences in characteristics (eg age, gender) between study groups were reported.

Data analysis
For each trial we calculated results in terms of odd ratios and their 95% CIs. Studies that were similar in terms of interventions were pooled by using Review Manager 5.2 software [18]. Heterogeneity was assessed with the I 2 test and Q statistics. An I 2 value greater than 50% and P < 0.1 was indicative of heterogeneity [19]. Random effects models were mainly used because of significant heterogeneity in some analyses. Figure 1 displays the flowchart of the study selection process [20]. A total of 1873 records were identified through database searches. Reference lists of related literature and systematic reviews were also searched for additional papers [2,7,9,16,[21][22][23]; Fourteen studies were identified, of which, one was a duplicate. Sixty-seven fulltext articles were retrieved to determine eligibility. Fortythree studies were excluded with the following reasons: no GP data (n = 31), not an RCT (n = 6), meta-analyses or literature review (n = 3), clinical trial (n = 3) and two articles could not be retrieved. A total of 23 studies were included in this review.

Results
We divided the strategies in two broad categories based on previous work conducted in this area: incentives and design based strategies [2]. There were 15 RCTs, four twoby-two factorial randomized design trials, one three-arm, Figure 1 Flowchart. two four-arm and one five-arm parallel randomized design trials (Table 1). There were three Australian, three American, one British and one Canadian study that measured the effectiveness of incentives. Nine Australian, three American, two Canadian, one Danish, and one Irish study included design based strategies. The majority of survey topics focussed on patient care, clinical guidelines and two studies [24,25] focussed on GPs personal work situation (eg job satisfaction). A summary of the risk of bias in included studies is presented in Table 2.

Monetary incentives
Four studies examined the effects of monetary incentives on GPs' response to surveys. Deehan and colleagues [11] found that cash payment and charity donation (£5 and £10) were more effective in increasing GP response rate compared to no incentive (pooled OR 1.87, 95% CI 1.48 to 2.36) ( Table 3). The study also demonstrated that cash incentives were more effective than charity donations compared to no incentive, and larger cash incentives were more effective than smaller cash incentives.
Asch and colleagues [26] also found that a larger cash incentive (US$5) was more effective than a smaller cash incentive (US$2) (OR 1.83, 95% CI 1.4 to 2.35). Everett and co-workers found that a $1 bill was more effective than no incentive. James and colleagues [27] found that upfront payment of a cash incentive (US$25) was significantly more effective than a promised cash incentive (US $25) (OR 2.88, 95% CI 1.70 to 4.89).
The extent of the risk of bias in all four studies examining monetary incentives is unclear as there was insufficient detail to determine if allocation of subjects were concealed, if participants and personnel were blinded to the study. In two studies it was not clear if there was absence of other bias. Missing data due to loss of follow up (eg undelivered surveys) was equally distributed between study groups in two studies [26,28], but this information was not reported in the other two studies [11,27]. Only James and co-workers [27] did not undertake an intention to treat analysis, which may have over estimated the effect of the intervention.

Nonmonetary incentives
Four studies [25,[29][30][31] examined the effects of nonmonetary incentives compared to no incentives on GP response rate to surveys. The weighted overall effect size showed a small but significant association between nonmonetary incentives and GP response (OR 1.39, 95% CI 1.15 to 1.69). Thomson et al. [32] compared two types of lottery incentives, and found that a lottery for one chance to win a large prize (6 bottles of champagne) was more effective than six chances to win one bottle (OR 1.47, 95% CI 1.05 to 2.08). All five studies that assessed nonmonetary incentives were randomised control trials, four undertook intention to treat analysis and all reported on pre-determined primary outcomes. The adequacy of allocation concealment and blinding of participants and personnel are unclear in four studies. Four studies did not report missing data due to loss to follow up, whilst only Robertson et al. [31] reported that missing data were equally distributed between study groups. No significant differences in demographic characteristics between study groups were found in Baron et al. [29] and Pit et al. [25], whilst this risk of bias was not clear in Robertson et al. [31], Thomson et al. [32] and Mclaren and Shelley [30].

Size of incentives
Three studies [11,26,32] found that a larger incentive had a small but significant effect on response rates (OR 1.47, 95% CI 1.19 to 1.81).
Finally, Ward and co-workers [33] used a pen as an incentive and compared it with other recruitment methods but did not find that a pen increased the response rates. They did find that women were more likely to respond to a pen.

Design based strategies
One study [34] found that asking general questions first do not significantly increase GP survey response rates.
Three studies [35][36][37] demonstrated that postal surveys are significantly more effective than telephone or email surveys (OR 1.82, 95% CI 1.19 to 2.78). Scott and colleagues [24] found that an online survey did not lead to increased response rates compared to using mixed methods. However, they found that sending a letter containing login details and an option to request a paper copy followed by a reminder that included login details and a paper copy led to increased response rates when compared to online surveys.
Two studies [34,38] found that pre-contact with GPs via a postcard or letter compared to no intervention increased the response rate but this was just not statistically significant (OR 1.16, 95% CI 0.99 to 1.37).
In terms of mode of the pre-contact, Gattellari et al. [13] found that pre-contact by fax was not statistically different to pre-contact by mail in increasing GP response rates. Gupta et al. [39] found that there was no statistical difference between pre-contact by a medical researcher compared to a non-medical researcher. But a study conducted by Ward and colleagues [33] demonstrated that an upfront phonecall from a peer led to increased response rates when compared to 3 other methods (pen, letter and research assistant prompt).
Bonevski and co-workers [40] found that reminder telephone calls to non-responders prior to the 3 rd mailout did not significantly increase response rates when compared to not conducting reminder telephone calls. Akl and co- To evaluate the cost effectiveness of a lottery on physicians response rates to a mail survey.

Family Doctors identified from membership list of the Quebec Federation of GPs
The determinants of influenza immunization among family physicians in Quebec.

RCT 1000
No lottery n = 500 Lottery n = 500 Canada Bonevskiet al., 2011 [40] To examine the efficacy of two strategies for improving general practitioner response to a survey and a secondary aim was to assess GPs self reported preferred mode of survey administration. To explore the effects of financial and non-financial inducements on response rates to increase the overall response rate.
A random sample of all GP's in England and Wales.
Survey on GP's work and attitudes with alcohol misusing patients.  To compare response rates to postal surveys sent by normal post and registered mail and to assess the cost implications of the two mailing methods.
General practitioners in Victoria.
To assess antenatal screening practices in Australia workers [41] found that tracking of responses did not help to improve response rates over not tracking responses. (Table 4) Personalisation of mail-out packages to GPs generally does not appear to increase response rates. Inclusion of a professional sponsorship letter, coloured seal with text and hand written thank you postscripts did not increase survey response rates [13,40,42]. However, Maheux et al. [42] found that a personalised mail-out package that included the physician's title, name and address individually typed onto the envelope, hand stamped outgoing envelopes identified to the university and hand stamped return envelope increased response rates with modest statistical significance (OR 1.73, 95% CI 1.04 to 2.87). Akl and colleagues [41] found that Friday morning was more effective than a Monday mailing. However, the pooled results of Akl et al. [41] and Olivarius and Andreasen [43] showed that timing of mailing at the end of the week compared to during the week does not effect survey response rates amongst GPs. Pedrana and coworkers [44] found that surveys sent by registered post significantly increased the response rates compared to surveys sent by standard post ( Table 4).
The majority of included studies that assessed the impact of design based strategies reduced the risk of bias by randomisation, reporting of predetermined primary outcomes and intention to treat analysis. A high risk of selection bias due to inadequate allocation concealment was evident in half (7 of 14) of these studies. Blinding of participants and personnel was undertaken in only one study, not undertaken in two of the studies and unclear in 11 studies.
Missing data were equally distributed between study groups in seven studies and unclear in the remaining seven studies. The balance of demographic characteristics between study subjects was demonstrated in five studies and unclear in the other eight studies ( Table 2).

Discussion
This review identified several strategies that can easily be implemented by researchers and policy makers to increase response rates for GP surveys. Monetary and nonmonetary incentives were more effective than no incentive. Large incentives were more effective than small incentives as were upfront monetary incentives compared to promised monetary incentives. Postal surveys were more effective than telephone or email surveys. One study demonstrated that sequentially mixed mode (online survey followed by a paper survey with the reminder) was more effective than an online survey or the combination of an online and paper survey sent similtaneously in the first mail-out. Precontact with a phonecall from a peer colleague, personalised packages, sending mail on Friday, and using registered mail also increased response rates in single studies. Precontact with a letter or postcard increased reponse rates slightly but just did not reach statistical significance.
This review focussed specifically on GPs to assist in increasing GP survey response which limits its generalisability to other doctor groups. There is evidence that GPs and specialists respond differently to recruitment strategies. For example, Robertson and colleagues found that incentives had a quantitatively larger effect amongst GPs compared to specialists response rates but this was not statistically significant [31]. Scott and co-workers [24] found that GPs had the lowest response rates and specialists the highest response rates. Specialists had statistically significant higher response rates compared to other doctors when offered an online and paper survey at the same time. On the contrary, Olivarius and Andreasen [43] concluded that GPs had higher response rates than specialists. Furthermore, Maheux and colleagues [42] reported that specialists were a little more sensitive to the interventions than the GPs. Lastly, James and colleagues found that there was no difference between doctor types and the impact of incentive type and timing on survey response rates [27]. The difference in reponse rates per type of doctor may well depend on a variety of factors including sociodemographic factors, the topic of interest and the level of involvement required to participate in the survey. This should be taken into account when designing recruitment strategies for surveys. We found substantial heterogeneity among the included studies which makes it difficult to generalise the study findings and determine the impact of other factors on the study results [3]. For example, in the general population more sensitive topics lead to lower survey response rates [8]. This was difficult to assess in this systematic review because there were not any study topics that were considered to be sensitive for GPs. In the study published by Seguin and colleagues in 2004 [37], the difference in response rate between postal and email surveys could be attributed to the year that the study was conducted because GPs may now be more used to email. A more recent study [24] found that online surveys led to lower response rates than mixed mode approaches (both online and paper based surveys). The reluctance of GPs to complete surveys online is confirmed in other studies [40]. Potential explanations are concerns about confidentiality, familiarity and access to the internet, and the ease of completing the survey online ( eg the need to recall or enter a password). It is possible that online response rates may improve in the future because of the growing use of smartphones and portable devices that make it easier and quicker to access online surveys. The number of reminders also may have had an impact on study results. The largest impact of incentives and other recruitment strategies is often seen in the first wave of recruitment and this steadily decreases with followup reminders [25,26].
Researchers must take into account the available resources when designing survey recruitment strategies. Shorter surveys, online surveys, the number of reminders and smaller incentives can all lead to costsavings but this needs to be carefully balanced with maximising high quality, valid and reliable data. Eleven studies in this systematic review included information on intervention costs [24,26,[29][30][31]34,36,38,39,41,44]. Three out of four studies which used nonmonetary incentives provided relevant cost information. Baron et al. [29], Mclaren and Shelley [30] and Robertson et al. [31] found that by offering a nonmonetary incentive at an additional cost of CAN$16, AU$3.46 and AU$23.42 per survey returned led to an increased response rate of respectively 6%, 4% and 15%. Asch and co-workers [26] found that a $5 incentive versus a $2 incentive cost only an additional $0.53 cents per survey returned with an increase in response rates of 15%.
Amongst design based studies, Scott and colleagues [24] found that the additional cost per 1% increase in response rate was AU$3290 for sequential mixed mode strategies and AU$10,156 for simultaneous mixed mode strategies when compared to the online mode. The additional cost per additional response was AU$6.07 for sequential mixed mode strategies and AU$18.75 for simultaneous mixed mode strategies with an increase in GP response rates of 6% and 2% respectively when compared to the online survey (response rate 14%). The sequential mixed mode strategy therefore appears to be the most cost-effective method. However, these cost figures need to be weighed up against the fact that there was some response bias in the sequential and simultenous mixed mode strategies and a higher rate of item non-response in the online survey. Researchers should be aware that there is some evidence that there are new differences between physician responders and non-responders and early and late responders, suggesting that there is a low level of nonresponse bias [2,14]. It is argued that the reason for this is that the population is rather homogeneous.
Little differences in cost per respondent were found in studies that used a letter or postcard as a pre-contact strategy. Drummond and co-workers [34] reported that sending a postcard led to an additional cost of €0.87 per respondent and an increase in the reponse rate of 4%. The non-postcard group had a response rate of 46% compared to 50% in the postcard group. Similarly, Pirotta et al. [38] found that sending a postcard added only AU$0.40 per respondent but increased the reponse by 6% from 60% to 66%. McLaren and co-workers [30] found no difference between telephone pre-contact and postcard pre-contact but that the costs involved in calling GPs was about 5 times as much as sending a postcard. Gupta and colleagues [39] found that pre-contact by a medical practitioner and a research assistant led to the similar costs because the research assistant had to make more calls and there was no statistical significant difference in response rates. It therefore appears that pre-contacting GPs by mail is a cheap method to improve response rates compared to doing nothing, and is cheaper than telephone pre-contact which may lead to similar response rates. Lastly, Pedrana and co-workers [44] found that sending surveys by registered mail cost an additional AU$1531.50 but led to a 19% increase in response rates (response rate: 86% registered mail versus 67% normal mail.). This seems to suggest that extra costs are justified in these cases.
In accordance with our study, systematic review studies amongst physicians have also found that monetary incentives increase response rates [2,14], as does pre-payment [2,14]. Contrary to our findings, Van Geest and Kellerman found that non-monetary incentives did not increase response rates amongst physicians [2,14]. This may also reflect differences between GPs and other physicians. VanGeest et al. [2] pointed out that non-monetary incentives are more likely to work if physicians value them. Large incentives were more effective than small incentives in our study and Kellerman and Herold's study [14]. However, Vangeest found that larger incentives led to mixed results for both monetary and non-monetary incentives [2].
Kellerman and Herold [14] found no difference between mail and telephone interviews, whereas our review did. This may be explained by the fact that our review included more recent studies. VanGeest also reported that family physicians prefer mail surveys [2]. The general consensus is that GPs still prefer mail surveys over online surveys. Pre-contact with a phonecall from a peer colleague, personalised packages, sending mail on Friday, and using registered mail also increased response rates in single studies. Kellerman and Herold [14] found that pre-notification was not effective but this conclusion was based on only one study. Kellerman and colleagues reported one study that found that personalised mailouts were effective during the 1 st mail out but no effect was found during 2 nd mailouts in two other studies. However, Maheux et al. [42] demonstrated an effect of personalised mailouts during 3 rd mailouts. This effect is likely to be explained by the fact that the researchers included multiple elements to personalise the package.
The quality of the included studies generally was not clear from current reporting. Allocation concealment and blinding of participants were clearly reported in only two studies. Complete outcome data were available for 10 studies. Absence of reporting bias was present in all studies, whereas absence of other bias was only applicable to 9 studies. The strength of this review is that , to our knowledge, this is the first systematic review that has examined GPs as a group and not a sample of multiple different medical practitioners. Another strength of this review is that the outcome measure is an objective outcome measure and is therefore less likely to be influenced by reporting bias within studies. A weakness of the study was that we did not approach authors for additional information due to the budget constraints. This has an impact on judging the quality assessment of the included studies as mentioned above, in particular in the area of blinding of participants and personel. Finally, a limitation of the study is that the majority of studies were conducted in English speaking countries and may therefore limit generalsibility to other countries.
Further research is required to advance this field of research. Areas that need further exploring include strategies and factors such as survey topic, confidentiality guarantees, level of incentives, use of social media, mobile phone applications, and sequential mixed method applications. Furthermore, better reporting is required to determine the quality of the included studies. We support Creavin and colleagues' suggestion to develop a standard template for survey studies, similar to the consort statement [3] to improve reporting on survey methodology. Finally, we also recommend that future studies measure and report on the resources used when conducting the interventions to guide future researchers in strategy selection. We recommend that primary care researchers can build randomised controlled trials into survey research to further test which strategies are most effective.

Conclusions
GPs response rates to surveys may improve by using the following strategies: monetary and nonmonetary incentives, larger incentives, upfront monetary incentives, postal surveys, pre-contact with a phonecall from a peer colleague, personalised packages, sending mail on Friday, and using registered mail. Mail pre-contact may also improve response rates and has low costs. Improved reporting and further trials, including mixed mode studies, are required to determine the effectiveness of recruitment strategies on general practitioners' response rates to surveys.

Additional file
Additional file 1: Search: Medline strategy.