 students, in this module we will talk about sample size estimation for unknown population means to say that when you don't know exactly how many of your population is or what is its exact number then how do we estimate the sample size come let's see for this formula we use when your population is unknown then it is called Z-score formula so in Z-score formula we estimate the sample size under this formula in this you have Z-score, standard deviation and sampling error so Z-score refers to the confidence interval if your 90% is Z-score then value 1.65 if 95% is Z-score then value 1.96 and if 99% is Z-score then value 2.226 standard deviation is 0.5 and E is the margin of error which is 0.05 so if we calculate its value with 90% confidence interval then the value we have is 268.96 i.e. your sample size will be 268 and if you keep value with 95% confidence interval then your value will be 384 i.e. your sample size will be 384 and if your value is 2.32 i.e. you keep 99% confidence interval then your sample size will be 541 i.e. as we have seen in previous formula that if your confidence interval increases then your sample size increases if your confidence interval decreases then your sample size decreases so same is the case here as well apart from this, when the population is unknown you have to conduct a study and there is another rule of thumb for this that you can see the sample size of previous studies in that domain that what was the sample size in that if your research questions are the same and your objectives are the same methodology is the same so whatever sample size was in those studies then you can take their average and keep that sample size and along with that we see that in qualitative studies that sample size decreases in quantitative studies in survey research sample size is high so it depends upon the nature of the research as well so this estimation of sample size we have seen for known and unknown population it is quite relevant for the quantitative studies and we try to follow this that our estimated sample size then we should reach out to it in data collection otherwise if we do not reach out to it then our response rate will decrease and the less the response rate will decrease the results of that will be questioned