# Help with null hypothesis and T-Statistics

#### jena2007

##### New Member
Hi ,

I'm having problems understanding null hypothesis and t-statistics. Especially this question(below):

12. You will have complete sales information for last month in a week, but right now you have data from a random sample of 40 stores. The mean change in sales in the sample is +3.8% and the standard deviation of the changes is 12%. Are average sales for all stores different from last month?

a. State appropriate null and alternative hypotheses. Explain how you decide between the one-and two-sided alternatives.
H0: µ = .038
Ha: µ ≠ .038

I used the two-sided alternative because it wants to know if the average sales for all stores are different from last month.

b. Find the t statistics and its P-value. State your conclusion.
t = X¯ - .038/(.12/√40 )= 2.00

P-value = .052

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#### jena2007

##### New Member
Hi,

I also have one more question:

If the test gives strong evidence against the null hypothesis, would you conclude that sales are up in every one of your stores? Explain your answer.

Is dependant on whether the p-value < or > α.

Thank You

#### jerryb

##### New Member
i think you want Ho = 0 in other words the null hypothesis is that no change has occurred.

cheers
jerry

#### JohnM

##### TS Contributor
jena2007 said:
If the test gives strong evidence against the null hypothesis, would you conclude that sales are up in every one of your stores? Explain your answer.
No - the inference helps you make statements regarding the average sales increase, but tells you nothing about individual stores.

E

#### elnaz

##### Guest
Hello
yes, your first answers are true ,but about second question if i have understood true , no .