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Support or Reject Null Hypothesis in Easy Steps

Hypothesis Testing > Support or Reject Null Hypothesis

Contents:

  1. What does it mean to reject the null hypothesis?
  2. Support or Reject the null hypothesis: Steps

What does it mean to reject the null hypothesis?

Watch the video or read the article below:

In many statistical tests, you’ll want to either reject or support the null hypothesis. For elementary statistics students, the term can be a tricky term to grasp, partly because the name “null hypothesis” doesn’t make it clear about what the null hypothesis actually is!

Overview

The null hypothesis can be thought of as a nullifiable hypothesis. That means you can nullify it, or reject it. What happens if you reject the null hypothesis? It gets replaced with the alternate hypothesis, which is what you think might actually be true about a situation. For example, let’s say you think that a certain drug might be responsible for a spate of recent heart attacks. The drug company thinks the drug is safe. The null hypothesis is always the accepted hypothesis; in this example, the drug is on the market, people are using it, and it’s generally accepted to be safe. Therefore, the null hypothesis is that the drug is safe. The alternate hypothesis — the one you want to replace the null hypothesis, is that the drug isn’t safe. Rejecting the null hypothesis in this case means that you will have to prove that the drug is not safe.

reject the null hypothesis
Vioxx was pulled from the market after it was linked to heart problems.

To reject the null hypothesis, perform the following steps:

Step 1: State the null hypothesis. When you state the null hypothesis, you also have to state the alternate hypothesis. Sometimes it is easier to state the alternate hypothesis first, because that’s the researcher’s thoughts about the experiment. How to state the null hypothesis (opens in a new window).

Step 2: Support or reject the null hypothesis. Several methods exist, depending on what kind of sample data you have. For example, you can use the P-value method. For a rundown on all methods, see: Support or reject the null hypothesis.

If you are able to reject the null hypothesis in Step 2, you can replace it with the alternate hypothesis.

That’s it!

When to Reject the Null hypothesis

Basically, you reject the null hypothesis when your test value falls into the rejection region. There are four main ways you’ll compute test values and either support or reject your null hypothesis. Which method you choose depends mainly on if you have a proportion or a p-value.

support or reject null hypothesis

Support or Reject the Null Hypothesis: Steps

Click the link the skip to the situation you need to support or reject null hypothesis for:

General Situations: P Value
P Value Guidelines
A Proportion
A Proportion (second example)

Support or Reject Null Hypothesis with a P Value


If you have a P-value, or are asked to find a p-value, follow these instructions to support or reject the null hypothesis. This method works if you are given an alpha level and if you are not given an alpha level. If you are given a confidence level, just subtract from 1 to get the alpha level. See: How to calculate an alpha level.

Step 1: State the null hypothesis and the alternate hypothesis (“the claim”).
If you aren’t sure how to do this, follow this link for How To State the Null and Alternate Hypothesis.

Step 2: Find the critical value. We’re dealing with a normally distributed population, so the critical value is a z-score.
Use the following formula to find the z-score.

null hypothesis z formula

Click here if you want easy, step-by-step instructions for solving this formula.

Step 4: Find the P-Value by looking up your answer from step 3 in the z-table. To get the p-value, subtract the area from 1. For example, if your area is .990 then your p-value is 1-.9950 = 0.005. Note: for a two-tailed test, you’ll need to halve this amount to get the p-value in one tail.

Step 5: Compare your answer from step 4 with the α value given in the question. Should you support or reject the null hypothesis?
If step 7 is less than or equal to α, reject the null hypothesis, otherwise do not reject it.

P-Value Guidelines

Use these general guidelines to decide if you should reject or keep the null:

If p value > .10 → “not significant
If p value ≤ .10 → “marginally significant”
If p value ≤ .05 → “significant”
If p value ≤ .01 → “highly significant.”

That’s it!

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Support or Reject Null Hypothesis for a Proportion

Sometimes, you’ll be given a proportion of the population or a percentage and asked to support or reject null hypothesis. In this case you can’t compute a test value by calculating a z-score (you need actual numbers for that), so we use a slightly different technique.

Sample question: A researcher claims that Democrats will win the next election. 4300 voters were polled; 2200 said they would vote Democrat. Decide if you should support or reject null hypothesis. Is there enough evidence at α=0.05 to support this claim?

Step 1: State the null hypothesis and the alternate hypothesis (“the claim”).
Ho:p ≤ 0.5
H1:p > .5

Step 2: Compute phat by dividing the number of positive respondents from the number in the random sample:
2200/4300 = 0.512.

Step 3: Use the following formula to calculate your test value.

test value with a proportion

Where:
Phat is calculated in Step 2
P the null hypothesis p value (.05)
Q is 1 – p

The z-score is:
.512 – .5 / √(.5(.5) / 4300)) = 1.57

Step 4: Look up Step 3 in the z-table to get .9418.

Step 5: Calculate your p-value by subtracting Step 4 from 1.
1-.9418 = .0582

Step 6: Compare your answer from step 5 with the α value given in the question. Support or reject the null hypothesis? If step 5 is less than α, reject the null hypothesis, otherwise do not reject it. In this case, .582 (5.82%) is not less than our α, so we do not reject the null hypothesis.

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Support or Reject Null Hypothesis for a Proportion: Second example

Sample question: A researcher claims that more than 23% of community members go to church regularly. In a recent survey, 126 out of 420 people stated they went to church regularly. Is there enough evidence at α = 0.05 to support this claim? Use the P-Value method to support or reject null hypothesis.

Step 1: State the null hypothesis and the alternate hypothesis (“the claim”). Ho:p ≤ 0.23; H1:p > 0.23 (claim)

Step 2: Compute phat by dividing the number of positive respondents from the number in the random sample:
63 / 210 = 0.3.

Step 3: Find ‘p’ by converting the stated claim to a decimal:
23% = 0.23.
Also, find ‘q’ by subtracting ‘p’ from 1: 1 – 0.23 = 0.77.

Step 4: Use the following formula to calculate your test value.

HYPOTHESIS test value with a proportion

Click here if you want easy, step-by-step instructions for solving this formula.

If formulas confuse you, this is asking you to:

  1. Subtract p fromphat(0.3 – 0.23 = 0.07). Set this number aside.
  2. Multiply p and q together, then divide by the number in the random sample. (0.23 x 0.77) / 420 = 0.00042
  3. Take the square root of your answer to 2. √(0.1771) = 0.0205
  4. Divide your answer to 1. by your answer in 3. 0.07 / 0.0205 = 3.41

Step 5: Find the P-Value by looking up your answer from step 5 in the z-table. The z-score for 3.41 is .4997. Subtract from 0.500: 0.500-.4997 = 0.003.

Step 6: Compare your P-value to α. Support or reject null hypothesis? If the P-value is less, reject the null hypothesis. If the P-value is more, keep the null hypothesis.
0.003 < 0.05, so we have enough evidence to reject the null hypothesis and accept the claim.

Note: In Step 5, I’m using the z-table on this site to solve this problem. Most textbooks have the right of z-table. If you’re seeing .9997 as an answer in your textbook table, then your textbook has a “whole z” table, in which case don’t subtract from .5, subtract from 1. 1-.9997 = 0.003.

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Support or Reject Null Hypothesis in Easy Steps was last modified: October 15th, 2017 by Stephanie

28 thoughts on “Support or Reject Null Hypothesis in Easy Steps

  1. Angie Widdows

    One thing I do not understand is why Ho:p ≤ 0.23. I would think it would be greater than or equal to because the question states “A researcher claims that more than”. Great. I am confused on the first step…

  2. Donna Allen

    I have to admit, this seemed a little overwhelming when first looking at all of the steps involved. But, your explanation was easy to follow and very helpful when doing the homework. Thank You.

  3. Lauren Schultz

    THIS HAS BY FAR BEEN MY MOST TRYING SECTION. AND ITS NOT EVEN THE WORK ITS THE LAST PART, THE WORDING. I GET SO LOST ON WETHER TO SUPPORT OR REJECT THE NULL. THE BOOK WORDS IT DIFF THAN THE HOMEWORK AND THE ‘HINT’ SECTION ON THE HOMEWORK IS NO HELP.
    IF ANYONE CAN HELP PLZ LET ME KOW ID BE FOREVER GREATFUL!! :)

  4. Donna Allen

    I agree that the wording is very confusing. I understand how to do the equations. But, the wording at the end of the problem is really confusing to me as well.

  5. Rebecca Gamble

    I agree with everyone else I don’t undestand the wording or anything about these hypothesis.

  6. Sarah M. Joyner

    I find these hypothesis word problems very confusing. I have done multiples problems and everything matches up except for my “yes” or “no” response. Maybe I am not understanding how to answer when you reject the hypothesis. It does not make since that you would then say yes to the claim????

  7. Alison Bryant

    I liked this one because it finally helped me to understand the P-value method, I was doing it backwards from how it should be understood in terms of Ha and Ho but now I understand.

  8. rachel

    i am confused why the The z-value for 3.41 is .4997. my table says the z value for 3.41 is .99996

  9. Stephanie

    You’re looking at the wrong z-table for the problem. There are two tables — one for the left of the curve and one for right of the curve.

  10. Saba

    This is very helpful for me, I finally understand how to answer such a question I the exam, but I don’t understand where the 0.500 is from
    And why to substract it from the z value ?!please clear this up for me as am
    Just learning about hypothesis testing, I’d also appreciate if you’d explain to me more about the z tables , are they like standard tables?! For all hypothesis testing ? Am a lil lost so please help!!:(

  11. Kris

    I’m a bit confused too. On my Standard Normal Distribution table, it shows the z-value for 3.41 to be .9997. Where do you get .4997 or .4977- either way I’m not seeing it.

  12. Andale

    Kris,

    There are a couple of different versions of the z-table. The sample problems on this site use the z-table that is right of z . *Some* stats books have the whole table…that’s probably what you are looking at. In any case, you would still get the same answer (1-.9999=.03). You just subtract from 1, not .5 if you are using a “whole z” table.

    Stephanie

  13. EZEKIEL

    It is known that the survival rate for individuals afflicted with a certain disease in a certain community is 90%. If 10 persons in the community contract the disease, what is the probability that (a) None will die (b) 50% will survive (c) at least 3 will die (d) exactly 3 will survive

    PLS HELP ME CALCULATE URGENTLY

  14. Andale

    Are you given any other information? For example, is your data normally distributed? Do you have a standard deviation?