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Table 1 Follow-up rates under varying assumptions estimated using four methods: (i) the standard Percentage Method (Eq. 1), (ii) the Clark’s Completeness Index (CCI, Eq. 2), (iii) the Person-Time Method estimated using the formal method (FPT, Eq. 4) and (iv) the Simplified Person-Time Method (SPT, Eq. 5)

From: New methods for estimating follow-up rates in cohort studies

Assumed event rate True Person-time follow-up rate
η PTFR
Percentage Method
η percentage
Estimated using the formal method
η FPT
Clark’s compleness inex
η CCI
Simplified Person-time method
η SPT
Average %bias1 \( \sqrt{\mathrm{MSE}} \) 2 Average %bias \( \sqrt{\mathrm{MSE}} \) Average %bias \( \sqrt{\mathrm{MSE}} \) Average %bias \( \sqrt{\mathrm{MSE}} \)
5% 95.0% 90.4% −4.90 .047 95.0% 0.00 .001 94.9% −0.08 .001 95.1% 0.05 .001
81.9% 66.7% −18.5 .158 82.1% 0.16 .002 81.7% −0.26 .003 82.1% 0.25 .003
68.1% 44.8% −34.3 .233 68.3% 0.45 .004 67.9% −3.49 .003 68.2% 0.59 .005
56.7% 29.3% −48.2 .274 57.3% 0.92 .006 56.5% −0.27 .003 57.3% 1.09 .007
10% 95.2% 91.0% −4.48 .043 95.2% −0.01 .002 95.1% −0.29 .003 95.4% 0.10 .002
82.1% 67.9% −17.4 .142 82.3% 0.17 .002 81.6% −0.69 .006 82.5% 0.49 .006
68.5% 46.5% −32.2 .220 68.9% 0.64 .005 67.9% −1.11 .008 69.2% 1.07 .009
56.4% 30.3% −46.2 .261 57.1% 1.33 .008 55.6% −1.36 .008 57.4% 1.84 .011
30% 94.4% 90.0% −4.61 .044 93.4% −0.95 .009 93.7% −0.64 .006 94.6% 0.31 .004
82.7% 70.7% −14.5 .120 82.3% −0.04 .004 81.1% −1.94 .016 83.6% 1.06 .009
69.4% 50.9% −26.8 .186 69.7% 0.42 .005 67.1% −3.33 .023 70.9% 2.17 .016
53.6% 31.0% −42.2 .226 54.8% 2.17 .012 51.1% −4.71 .026 55.9% 4.19 .023
50% 93.2% 89.5% −3.97 .037 88.3% −0.53 .049 91.2% −2.08 .020 93.9% 0.80 .008
77.6% 67.2% −13.4 .105 74.6% −3.79 .030 72.5% −6.58 .051 79.9% 3.00 .024
65.0% 51.1% −2.14 .140 63.6% 2.03 .014 58.5% −9.93 .647 68.4% 5.25 .035
46.6% 31.3% −32.8 .153 47.8% 0.03 .014 40.0% −14.0 .066 51.3% 10.0 .005
  1. These results were compared to the true Person-time follow-up Rate (Eq. 3) based on complete information generated under the simulations, each averaged across 1000 simulated data sets. The simulations involved an assumed 5-year prospective cohort study of N = 1000 subjects with fixed annual interval clinical visits and non-informative dropout. Time-to-event was generated based on exponential distributions with event rates varied from 5 to 50% and time to dropout was generated based on an independent exponential distribution with dropout proportion varying from 10 to 70%. Results were averaged across the 1000 simulated datasets
  2. Note: 1. % bias was calculated as (average of the particular method-η PTFR )/η PTFR *100%; 2. \( \sqrt{\mathrm{MSE}} \) was calculated as the square root of the average of (estimate-η PTFR )2. MSE from the true η PTFR was calculated instead of variance because several methods used here can be biased