Sample size determination

Dear Dr. Stats,

Good day.

I would like to conduct a PhD research on Certification, Recertification and Implementation of Preschool Vision screening in Malaysia but encountered problems in calculating sample size.

From the earlier work done by Master candidate, they had trained 180 preschool teachers and 30 preschool supervisors to carry out vision screening for preschoolers.

Phase I of my project is to determine the level of competency in carrying out the screening among preschool teachers after 2 years of training , as to determine the necessity of recertification module. So the total sample size I should take should be 180 (all trained preschool teachers) ?? We are testing them by quantitative assessments (written theory and practical exam) and qualitative assessment (face-to-face interview). For practical exam, should I calculate the sample size for examiners??

Phase II is to held a focused group discussion with the relevant stakeholders after find out there is a necessity for recertification to improve the current module in certification for vision screening after gathering all the earlier information and professionals opinion from colleagues.

Phase III is to conduct the improvised Certification module to preschool teachers via pilot study, by comparing the effectivity of delivery of the certification module by preschool supervisors (case group) and by optometrist (control group). How should I calculate the sample size for the pilot study? Most literature said take 10% from the actual sample for pilot study, means 10% of 30 preschool supervisors which is equal to 3 supervisors?? so 3 supervisors and 3 optometrists?? and how about the sample size for preschool teachers to be trained by both case and control group? can i use the formula for comparing 2 independent proportions which will give 30 preschool teachers for each group? So in case group, 3 supervisors trained 30 teachers ;In control group, 3 optometrist trained 30 teachers? And how many preschool children need to be tested for each teacher?

Phase IV is the Certification and Recertification of preschool vision screening which is conducted by preschool supervisors to teachers if their results showed compatible with optometrist. If not, optometrist will need to conduct the training to preschool teachers. To determine the sample size for supervisors, can I use formula for known sample size (known sample size = 30) by Krejcie & Morgan? and how to determine the sample size for preschool teachers and preschool children?

Phase V is to refer those children who failed in the vision screening to Optometry Clinic for comprehensive evaluation & management.

Sorry for sudden questions by me. Appreciate if you could offer yourself in assisting these issues.


Many Many thanks & regards,
Tan Xuan Li
+6017 321 1317
[email protected]
Postgraduate student
Department of Optometry & Vision Science
Faculty of Health Sciences
National University of Malaysia