When analyzing the iron content of a well drilled for use in a new housing development, water from the well was analyzed on three different days. The following data was obtained:
Sample Number of Fe content
measurements mg/L
on the sample
1 3 1.56, 1.45, 1.51
2 3 1.42, 1.39, 1.46
3 3 1.52, 1.29, 1.41
Calculate the pooled estimate of the standard deviation of the analysis.