Table 1 Level of evidence for a representative sample (high, med, low, unclear)

High

‘High’ level of evidence for a representative sample (or ‘low risk’ of selection bias).

1.The investigators describe a clear, defined and reliable sample frame for the target group

 •Sample frame already known e.g. hidden group reliably detected in census data or registry

 •Sample frame created by the researchers and includes the vast majority of the hidden target group

2.The investigators describe a random component in the process of drawing from the sample frame such as:

 •Referring to a random number table;

 •Using a computer random number generator

*Note: Must fulfil both 1 and 2 criterion for a judgement to be made of ‘high’ level evidence for representativeness.

Medium

‘Medium’ level of evidence for a representative sample (or ‘medium risk’ of selection bias)

Sampling frame and sampling processes are applied from both the high and low below criteria.

Non-random sampling with

 •Multiple efforts and techniques used with the a priori aim to approximate a representative sample including two or more of:

  oRespondent driven sampling (RDS)

  oQuota sampling

  oMaximum variation

  oMultiple starting points for snowballing

  oSample representativeness ascertained e.g. sample compared with census demographics of the hidden group.

Low

‘Low’ level of evidence for a representative sample (or ‘high risk’ of selection bias)

1.The investigators do not use a comprehensive sample frame for the hidden target group

 •Sample frame likely to exclude a significant proportion of the target group

2.The investigators describe a non-random component in the sampling process, for example:

 •Snowballing

 •Convenience sampling

3.Low sample numbers

4.Low response rates

Unclear

‘Unclear’ evidence for a representative sample (or unclear selection bias).

Indicates a lack of information about the sample frame and sample drawn.