In laboratory work. it is desirable to run careful checks on the variability of readings produced on standard samples. In a study of the amount of calcium in drinking water undertaken as part of a water quality assessment. the same standard sample was run through the laboratory six times at random intervals. The six readings. in parts per million. were 9.54, 9.61, 9.32. 9.48, 9.70. and 9.26:. Estimate the population variance :12 for readings on this standard, using a 90% condence interval