In our text, we state that the variance of N observations, x₁, x₂, . . . , x_N (when N is large), for a numeric attribute X is defined as

where X̅ is the mean value of the observations, as defined in Eq. (??). This is actually the formula for calculating the variance for the whole population using all the data (hence called the population variance). If we are calculation the variance using only a sample of data (hence called sample variance), we will need to use the following formula

where n is size of the sample. With the sample size n, sample standard deviation can defined similarly. Explain why there is such a minor difference at defining sample variance and population variance.

Transcribed Image Text:

N 2 -() - N i=1 N 1 # - *)2. i=1 72,