Should the formula for calculation of the sample size include the expected response rate? If I am not wrong, an existing formula implies 100% response rate! What if my expected response rate is 10%? I have population N=33500 and my calculated sample size is 380 (confidence level of 95% with a margin of error of 5%). If my expected response rate is 10% should I sent an email invitation to 3800 persons to make sure that I will have 380 responses? Another question is about randomness of my sample. I can randomly chose the 3800 potential participants but my sample still will not be random duo to the non-response bias. Is there any way to make sure that sample is really random?