Question

Suppose that a recent article stated that the mean time spent in jail by a first-time convicted burglar is 2.5 years. A study was then done to see if the mean time has increased in the new century. A random sample of 27 first-time convicted burglars in a recent year was picked. The mean length of time in jail from the survey was four years with a standard deviation of 1.9 years. Suppose that it is somehow known that the population standard deviation is 1.4. Conduct a hypothesis test to determine if the mean length of jail time has increased. Assume the distribution of the jail times is approximately normal.

Answer 1

sigma should be used

Explanation:

Given that The mean length of time in jail from the survey was four years with a standard deviation of 1.9 years.

The above given is for sample of 27 size.

For hypothesis test to compare mean of sample with population we can use either population std dev or sample std dev.

But once population std deviation is given, we use only that as that would be more reliable.

So here we can use population std deviation 1.4 only.

If population std deviation is used we can use normality and do Z test

Hope this helps ;)