Pooled variance is a method for estimating variance given several different samples taken in different circumstances where the mean may vary between samples but the true variance (equivalently, precision) is assumed to remain the same.
A combined variance is a method for estimating variance from several samples, given the size, mean and standard deviation of each. Mathematically, a combined variance is equal to the calculated variance of the set of the data from all samples.
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When data is homogeneous over k independent samples of size n_i for i=1,2,...,k, the pooled variance is given by s_p^2=((n_1-1) s_1^2+(n_2-1) s_2^2+⋯(n_k-1) s_k^2)/(n_1+n_2+⋯+n_k-k)
it means to rebulid
1- Assuming this represents a random sample from the population, the sample mean is an unbiased estimator of the population mean. 2-Because they are robust, t procedures are justified in this case. 3- We would use z procedures here, since we are interested in the population mean.
equal variances
is a data set with same variables but observed from different group of respondents