Answer :
Testing the hypothesis, we have that:
a)
The null hypothesis is: [tex]H_0: \mu = 128[/tex]
The alternative hypothesis is: [tex]H_1: \mu \neq 128[/tex]
b) The p-value of the test is of 0.1212.
c) Since the p-value of the test is of 0.1212 > 0.05, we cannot conclude that the mean daily number of texts for 25–34 year olds differs from the population daily mean number of texts for 18–24 year olds.
d) Since |z| = 1.55 < 1.96, we cannot conclude that the mean daily number of texts for 25–34 year olds differs from the population daily mean number of texts for 18–24 year olds.
Item a:
At the null hypothesis, we test if the mean is the same, that is, of 128 texts every day, hence:
[tex]H_0: \mu = 128[/tex]
At the alternative hypothesis, we test if the mean is different, that is, different of 128 texts every day, hence:
[tex]H_1: \mu \neq 128[/tex]
Item b:
We have the standard deviation for the population, thus, the z-distribution is used. The test statistic is given by:
[tex]z = \frac{\overline{x} - \mu}{\frac{\sigma}{\sqrt{n}}}[/tex]
The parameters are:
[tex]\overline{x}[/tex] is the sample mean.
[tex]\mu[/tex] is the value tested at the null hypothesis.
[tex]\sigma[/tex] is the standard deviation of the population.
n is the sample size.
For this problem, the values of the parameters are: [tex]\overline{x} = 118.6, \mu = 128, \sigma = 33.17, n = 30[/tex]
Hence, the value of the test statistic is:
[tex]z = \frac{\overline{x} - \mu}{\frac{\sigma}{\sqrt{n}}}[/tex]
[tex]z = \frac{118.6 - 128}{\frac{33.17}{\sqrt{30}}}[/tex]
[tex]z = -1.55[/tex]
Since we have a two-tailed test, as we are testing if the mean is different of a value, the p-value is P(|z| < 1.55), which is 2 multiplied by the p-value of z = -1.55.
Looking at the z-table, z = -1.55 has a p-value of 0.0606
2(0.0606) = 0.1212
The p-value of the test is of 0.1212.
Item c:
Since the p-value of the test is of 0.1212 > 0.05, we cannot conclude that the mean daily number of texts for 25–34 year olds differs from the population daily mean number of texts for 18–24 year olds.
Item d:
Using a z-distribution calculator, the critical value for a two-tailed test with 95% confidence level is |z| = 1.96.
Since |z| = 1.55 < 1.96, we cannot conclude that the mean daily number of texts for 25–34 year olds differs from the population daily mean number of texts for 18–24 year olds.
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