Thanks a lot Dr.Sabri for your advice.

Hi Mohammed,

Use the Attrition Formula:

(Number to enroll (N) = estimated sample size (n) / % retained (y) which in your case should be—

N = 371 / 0.65 = 571

The number of questionnaires to distribute is 571

[Always assume the worst possible scenario whenever you’re doing sample size calculation. In this case, 65% (0.65) is the worst case scenario. The bottom line is: you’re aiming to retain at least 371 sample size. If you get more, even better].

Thanks a lot Dr. Mohammed alhaji

I recommend Dr. Alhaji answer.

It would be better if attache the formula reference.

Hi Ali Nasseer,

The attrition formula can be found on this refreence:

1) Sullivan, L. (n.d.). Power and Sample Size Determination. Retrieved December 13, 2016, from http://sphweb.bumc.bu.edu/otlt/MPH-Modules/BS/BS704_Power/BS704_Power_print.html

For participation/attrition rates:

2) http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.571.5868&rep=rep1&type=pdf

Noted with thanks Mr. Mohammed.

Much appreciaed . Best Helen

If I were you I would also refer to G*Power's results that considers the case from different aspects as well. Just Google it!

Thanks a lot Dr.Sabri for your advice.

Although the calculation of Mr. Alhaji is pretty fine but my recommendation is, as a precautionary measure, please distribute more questionnaires than 571. The reason is that this calculation is just an "estimate" and taking new responses from new population or even from already studied population does not ensure that you will get the same number of responses.

Good luck!

Dear Mohammed Abdulrab,

Asking a good question is also art. I am waiting to read the response that will be received in this forum.

good question

I follow Dr.Mohammed M.Al- Haji.

best regard

have to use 498 as your sample size keeping 38% attrition in mind with a confidence level of 95 % and expecting your estimate will have an error of 5%(absolute error)

I am agree with Dr. Alhaji answer

Also

I am agree with Dr. Alhaji answer

oF