AP Statistics : Inference

Study concepts, example questions & explanations for AP Statistics

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Example Questions

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Example Question #1 : Inference

We are comparing the Democratic percentage of Detroit to the Democratic percentage of Dallas. 

In Detroit, we sampled 300 people and 208 were Democrats (.693)

In Dallas, we sampled 350 people and 265 were Democrats (.757)

What is the p-value for the .064 difference?

(Assume a 2-tailed test.)

Possible Answers:

.0244

.0688

.0344

.0577

.0765

Correct answer:

.0688

Explanation:

Overall percentage =

.0344 is for one tail, so .0688 is for both tails.

(Percentages are different at the .9312 level.)

Example Question #1 : Significance

      

       

Possible Answers:

       

       

       

Correct answer:

       

Explanation:

    

Example Question #1 : How To Test Of Significance For The Slope Of A Least Squares Regression Line

A statistician conducts a regression analysis and obtains a p-value of 0.1. It is more likely than not that there is a relationship between the variables in the study.

Possible Answers:

True

False

Correct answer:

True

Explanation:

A p-value of 0.1 is generally not sufficient to reject the null hypothesis, but this is only because we want a high degree of confidence before finding a relationship between variables.  Here, there is most likely a relationship between the variables even though the statistician could not reject the null hypothesis.

Example Question #1 : How To Test Of Significance For The Slope Of A Least Squares Regression Line

For a data set, the least-squares regression line has a  confidence interval for the slope of .

Based on this confidence interval, what can you do with a hypothesis test at  significance level where and ?

Possible Answers:

Fail to reject the null hypothesis because this confidence interval does not include .

Fail to reject the null hypothesis because this confidence interval does not include .

Not enough information to be able to decide.

Reject the null hypothesis because this confidence interval does not include .

Reject the null hypothesis because this confidence interval does not include .

Correct answer:

Reject the null hypothesis because this confidence interval does not include .

Explanation:

Notice that the interval does not include . This means that the P-value for the hypothesis test would be under 5%, which would lead us to reject our null hypothesis.

Any confidence interval can be used to create a hypothesis test by inverting it, and it is fairly simple, but the concept is tested into graduate-level statistics theory.

Example Question #1 : How To Test Of Significance For The Slope Of A Least Squares Regression Line

Which of the following is an incorrect condition requirement for regression inference?

Possible Answers:

Ordered pairs must be independent of each other

Response must vary normally about the regression line for any given value of

A linear relationship between  and 

A trend/pattern of some sort in the residual plot

The standard deviation of the response must be constan

Correct answer:

A trend/pattern of some sort in the residual plot

Explanation:

All of the following choices are correct conditions except for the choice concerning a trend/pattern of some sort in the residual plot. For regression inference to be accurate, we need to look at the residual plot of the data of interest and make sure there is random scatter. Random scatter indicates that the ordered pairs are indeed independent of each other. Any sort of pattern present in the residual plot would not satisfy that requirement, and therefore would not enable us to successfully use regression inference.

Example Question #1 : How To Do Tests Of Significance For A Mean

    

     

Possible Answers:

      

      

     

Correct answer:

     

Explanation:

      

Example Question #2 : Inference

Suppose you conduct a paired -test to assess whether two group means significantly differ and find a -score of 1.645. At what alpha level would this cause you to reject the null?

Possible Answers:

Correct answer:

Explanation:

The critical value for a -test with alpha set to 0.10 is 1.645.

Example Question #1 : Mean And Linear Regression

Bob wants to statistically determine if the mean height of middle school boys is greater than the mean height of middle school girls. He wants to use a significance level of  What must be true for him to reject . is known.

Possible Answers:

Correct answer:

Explanation:

Step 1: We need to use a 2-sample z test because there are 2 samples, boys and girls. The population standard deviation, , is known, so we can assume a standard distribution for each sample.

Step 2: This is a one-sided z test because the questions asks if the mean height of boys is greater than the mean height of girls.

Step 3: significance level, or alpha, is . This means we need a p-value less than  in order to reject the null hypothesis . A z-score greater than would ensure a p-value less than .

 

Curve p value

Example Question #1 : How To Define A Type Ii Error

If a test has a power of , what is the probability of Type II error?

Possible Answers:

Correct answer:

Explanation:

From the statistical definition of power (of a test), the power is equal to  where  represents the Type II error.

Therefore our equation to solve becomes:

Example Question #1 : How To Define A Type Ii Error

You and a classmate wanted to test the effect of sugars and fats on levels of blood sugar.

Your classmate told you that they found the null hypothesis valid, which was what there is no difference between the effects of sugars and fats on blood sugar levels.

If the null hypothesis was actually false, what type of error was made?

Possible Answers:

Neither

Type I and II

Type II

Type I

Correct answer:

Type II

Explanation:

A type I error occurs when the null hypothesis is valid but rejected.

A type II error occurs when the null hypothesis is false, but fails to be rejected.

Because the null hypothesis was false, but had failed to be rejected, they made a Type II error.

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