Owing to a change in classification procedures in the Birth Registry in 1978, there was more missing data on gestational age for the years 1978 1981 than for other years (mean missing proportion, 21.7% for 1978 1981, compared with 2.4% for the years 1973 1977, and 0.7% for the years 1982 2002).
Missing proportions ranged from 0.1% for seasonal flu vaccination history to 5.5% for the item "I have planned when and where to get my pH1N1 vaccination this winter".
This assessment included comparison of the national survey protocols with the EHES protocol, evaluation of the proportion of missing data for each data item, proportion of identical sequential readings and the proportion of terminal digits.
Strengths include large sample size, prospective design, and virtually complete follow-up; however, histologic details were missing for a proportion of excised tumors.
The proportion missing for pH was 23.4 %.
The proportion missing for participant economic activity was also high (6% Boost, 0.3% Core).
When these two categories were excluded, the proportion missing for stage was 63%.
Despite the presence of guidelines and recommendations for clear documentation around the reporting of missing data [ 1- 3, 5, 9, 10, 30, 31], there remains poor reporting of the amount of missing data with only half of the articles in this review providing the proportion of the complete cases or the proportion of missing data for each variable of interest in the manuscript.
Missing data were not computed because the overall proportion of missing responses for the HbA1c at 18 months was small (7%, 33/473) and the proportion was similar in the two study groups; 6.4% (15/236) in the intervention group and 7.6% (18/237) in the control group.
The variety of datasets allowed the procedures to be investigated for different proportions of missing data and for different missing data patterns.
