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What is a Null Hypothesis? - Definition & Examples

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  • 0:00 What Is a Null Hypothesis?
  • 1:08 What Is an Alternative…
  • 1:55 Examples
  • 3:01 Lesson Summary
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Lesson Transcript
Instructor
Karin Gonzalez

Karin has taught middle and high school Health and has a master's degree in social work.

Expert Contributor
Robert Ferdinand

Robert Ferdinand has taught university-level mathematics, statistics and computer science from freshmen to senior level. Robert has a PhD in Applied Mathematics.

This lesson will give the definition of a null hypothesis, as well as an alternative hypothesis. Examples will be given to clearly illustrate the concept of a null hypothesis versus an alternative hypothesis.

What Is a Null Hypothesis?

A hypothesis is a speculation or theory based on insufficient evidence that lends itself to further testing and experimentation. With further testing, a hypothesis can usually be proven true or false. Let's look at an example. Little Susie speculates, or hypothesizes, that the flowers she waters with club soda will grow faster than flowers she waters with plain water. She waters each plant daily for a month (experiment) and proves her hypothesis true!

A null hypothesis is a hypothesis that says there is no statistical significance between the two variables in the hypothesis. It is the hypothesis that the researcher is trying to disprove. In the example, Susie's null hypothesis would be something like this: There is no statistically significant relationship between the type of water I feed the flowers and growth of the flowers. A researcher is challenged by the null hypothesis and usually wants to disprove it, to demonstrate that there is a statistically-significant relationship between the two variables in the hypothesis.

What Is an Alternative Hypothesis?

An alternative hypothesis simply is the inverse, or opposite, of the null hypothesis. So, if we continue with the above example, the alternative hypothesis would be that there IS indeed a statistically-significant relationship between what type of water the flower plant is fed and growth. More specifically, here would be the null and alternative hypotheses for Susie's study:

Null: If one plant is fed club soda for one month and another plant is fed plain water, there will be no difference in growth between the two plants.

Alternative: If one plant is fed club soda for one month and another plant is fed plain water, the plant that is fed club soda will grow better than the plant that is fed plain water.

Examples

Example 1:

Null Hypothesis: The Earth is flat.

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Additional Activities

Null Hypothesis: Questions for Additional Practice


1. The average life of a car battery of a certain brand is five years. This information is gathered using data obtained from people who have purchased and used this brand of battery over a period of several years. A researcher at the battery company develops a new type of car battery and claims that the average life of this battery is more than five years. To determine whether this claim is true, one would need to do some hypothesis testing. What would be the null hypothesis and the alternative hypothesis for this hypothesis test?

2. Past research data from a period of over several years states that the average life expectancy of whales is 100 years. A researcher at a laboratory wishes to test this hypothesis. To that end they procure a sample of life spans of several whales. What is the null hypothesis and the alternative hypothesis that this researcher will establish?


Solutions

1. The researcher's null hypothesis will be: ''The average life of the new car battery is five years.''

Then their alternative hypothesis will read: ''The average life of the new car batter is more than five years.''


2. The null hypothesis for the researcher will state that, ''The average life expectancy of whales is exactly equal to 100 years.''

Their alternative hypothesis will read: ''The average life expectancy of whales is not equal to 100 years.''

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