Using the t-distribution, the most appropriate conclusion for the hypotesis test is given by Phone use did not change.

How to find the value of z-statistic for population mean? (z test statistic)

Suppose we're specified that:

The sample mean = [tex]\overline{x}[/tex]

The population mean =[tex]\mu[/tex]

The population standard deviation = [tex]\sigma[/tex]

The sample size = n

Then the z-statistic for this data is found as:

[tex]Z = \dfrac{\overline{x} - \mu}{\sigma/\sqrt{n}}[/tex]

In this problem, the values of those parameters are as follows:

The sample mean [tex]\overline{x}[/tex] = -0.2

The population mean =[tex]\mu[/tex] = 0

The population standard deviation = [tex]\sigma[/tex] = 9.1

The sample size = n = 200

Hence, the test statistic is given by:

[tex]Z = \dfrac{\overline{x} - \mu}{\sigma/\sqrt{n}}\\\\\\Z = \dfrac{-0.2- 0}{9.1/\sqrt{200}}\\\\z = 0.31[/tex]

Thus,

t = 0.31.

Since the absolute value of the test statistic is less than the critical value, we do not reject the null hypothesis and the conclusion is:

Phone use did not change.

Learn more about z-statistic here: