A nondirectional alternative hypothesis states that the null hypothesis is wrong. A nondirectional alternative hypothesis does not predict whether the parameter of interest is larger or smaller than the reference value specified in the null hypothesis.

A directional alternative hypothesis states that the null hypothesis is wrong, and also specifies whether the true value of the parameter is greater than or less than the reference value specified in null hypothesis.

The advantage of using a directional hypothesis is increased power to detect the specific effect you are interested in. The disadvantage is that there is no power to detect an effect in the opposite direction.

Example of directional and nondirectional hypothesesNondirectional

A researcher has results for a sample of students who took a national exam at a high school. The researcher

wants to know if the scores at that school differ from the national average of 850. A nondirectional alternative hypothesis is appropriate because the researcher is

interested in determining whether the scores are either less than or greater than the national average. (H0: μ =

850 vs. H1: μ≠ 850)Directional

A researcher has exam results for a sample of students who took a training course for a national exam. The researcher wants to know if trained students score

above the national average of 850. A directional alternative hypothesis can be used because the

researcher is specifically hypothesizing that scores for trained students are greater than the national average.

(H0: μ = 850 vs. H1: μ > 850)

ActivityChange the following questions into hypothesis using the

If...then format.

1. How does the amount of daylight affect the number of eggs laid by a chicken?

2. How does classroom temperature affect students' performance in a test?

3.How will batting practice affect a players' batting average?

4. How does advertising affect the sales of Orbit White?

5. Does adding fertilizer affect the growth of a plant?

Name: ____________________________For each item below, specify the independent and dependent variables, as well as constants.

1. A study was done to find if different tire treads affect the braking distance of a car.I: _________________ D: ___________________ C: __________________

2. The time it takes to run a mile depends on the person’s running speed.I: ________________ D: ___________________ C: __________________

3. The height of bean plants depends on the amount of water they receive.I: ________________ D: ___________________ C: __________________

4. The higher the temperature of the air in the oven, the faster a cake will bake.I: ________________ D: ___________________ C: __________________

5. Lemon trees receiving the most water produced the most lemons.I: ________________ D: ___________________ C: __________________

6. An investigation found that more bushels of potatoes were produced when the soil wasfertilized more.

I: ________________ D: ___________________ C: __________________7. Students measured the temperature of the water at different depths in Lake Skywalker andfound that the temperature varied.

I: ________________ D: ___________________ C: __________________8. The amount of pollution produced by cars was measured for cars using gasoline containingdifferent amounts of lead.

I: ________________ D: ___________________ C: __________________9. Four groups of rats are first massed and then fed identical diets except for the amount ofvitamin A they receive. Each group gets a different amount. After 3 weeks on the diet, the rats’masses are measured again to see if there has been a decrease.

I: ________________ D: ___________________ C: __________________