Question

The manager of a chain of pizza restaurants monitors operators taking orders by phone. The average time for a phone call, in seconds, is used as one summary of activity for a restaurant. Phone calls that take more than 4 minutes are considered to be inefficient since they tend to tie up phone lines and in the long run result in a significant reduction in revenue. The talk times (in seconds) of a random sample of operators are:

195 223 230 237 271 239

275 262 226 275 279 214

176 208 189 245 199 200

302 222 189 223 285 275

235 190 223 300 276 263

280 199 245 279 195

n= 35

sum of all= 8324

sum of each squared=2024658

a) Estimate with a 90% confidence interval, the average talk time for all orders taken by phone. Interpret your answer.

I calculated the interval with a T value because this is a sample and we do not know the real standard deviation of the whole population. However, the teacher used a Z value. Why?

Answer 1