A step-by-step guide to hypothesis testing
Hypothesis testing is a formal procedure for investigating our ideas about the world using statistics. It is most often used by scientists to test specific predictions, called hypotheses, that arise from theories.
There are 5 main steps in hypothesis testing:
- State your research hypothesis as a null (Ho) and alternate (Ha) hypothesis.
- Collect data in a way designed to test the hypothesis.
- Perform an appropriate statistical test.
- Decide whether the null hypothesis is supported or refuted.
- Present the findings in your results and discussion section.
Though the specific details might vary, the procedure you will use when testing a hypothesis will always follow some version of these steps.
Step 1: State your null and alternate hypothesis
After developing your initial research hypothesis (the prediction that you want to investigate), it is important to restate it as a null (Ho) and alternate (Ha) hypothesis so that you can test it mathematically.
The alternate hypothesis is usually your initial hypothesis that predicts a relationship between variables. The null hypothesis is a prediction of no relationship between the variables you are interested in.
You want to test whether there is a relationship between gender and height. Based on your knowledge of human physiology, you formulate a hypothesis that men are, on average, taller than women. To test this hypothesis, you restate it as:
Ho: Men are, on average, not taller than women.
Ha: Men are, on average, taller than women.
Step 2: Collect data
For a statistical test to be valid, it is important to perform sampling and collect data in a way that is designed to test your hypothesis. If your data are not representative, then you cannot make statistical inferences about the population you are interested in.
To test differences in average height between men and women, your sample should have an equal proportion of men and women, and cover a variety of socio-economic classes and any other variables that might influence average height.
You should also consider your scope (Worldwide? For one country?) A potential data source in this case might be census data, since it includes data from a variety of regions and social classes and is available for many countries around the world.
Step 3: Perform a statistical test
There are a variety of statistical tests available, but they are all based on the comparison of within-group variance (how spread out the data is within a category) versus between-group variance (how different the categories are from one another).
If the between-group variance is large enough that there is little or no overlap between groups, then your statistical test will reflect that by showing a low p-value. This means it is unlikely that the differences between these groups came about by chance.
Alternatively, if there is high within-group variance and low between-group variance, then your statistical test will reflect that with a high p-value. This means it is likely that any difference you measure between groups is due to chance.
Your choice of statistical test will be based on the type of data you collected.
Based on the type of data you collected, you perform a one-tailed t-test to test whether men are in fact taller than women. This test gives you:
- an estimate of the difference in average height between the two groups.
- a p-value showing how likely you are to see this difference if the null hypothesis of no difference is true.
Your t-test shows an average height of 175.4 cm for men and an average height of 161.7 cm for women, with an estimate of the true difference ranging from 10.2cm to infinity. The p-value is 0.002.
Step 4: Decide whether the null hypothesis is supported or refuted
Based on the outcome of your statistical test, you will have to decide whether your null hypothesis is supported or refuted.
In most cases you will use the p-value generated by your statistical test to guide your decision. And in most cases, your cutoff for refuting the null hypothesis will be 0.05 – that is, when there is a less than 5% chance that you would see these results if the null hypothesis were true.
Step 5: Present your findings
The results of hypothesis testing will be presented in the results and discussion sections of your research paper.
In the results section you should give a brief summary of the data and a summary of the results of your statistical test (for example, the estimated difference between group means and associated p-value). In the discussion, you can discuss whether your initial hypothesis was supported or refuted.
In the formal language of hypothesis testing, we talk about refuting or accepting the null hypothesis. You will probably be asked to do this in your statistics assignments.
Stating results in a statistics assignment
In our comparison of mean height between men and women we found an average difference of 14.3cm and a p-value of 0.002; therefore, we can refute the null hypothesis that men are not taller than women and conclude that there is likely a difference in height between men and women.
However, when presenting research results in academic papers we rarely talk this way. Instead, we go back to our alternate hypothesis (in this case, the hypothesis that men are on average taller than women) and state whether the result of our test was consistent or inconsistent with the alternate hypothesis.
If your null hypothesis was refuted, this result is interpreted as being consistent with your alternate hypothesis.
Stating results in a research paper
We found a difference in average height between men and women of 14.3cm, with a p-value of 0.002, consistent with our hypothesis that there is a difference in height between men and women.
These are superficial differences; you can see that they mean the same thing.
You might notice that we don’t say that we accept or reject the alternate hypothesis. This is because hypothesis testing is not designed to prove or disprove anything. It is only designed to test whether a pattern we measure could have arisen by chance.
If we reject the null hypothesis based on our research (i.e., we find that it is unlikely that the pattern arose by chance), then we can say our test lends support to our hypothesis. But if the pattern does not pass our decision rule, meaning that it could have arisen by chance, then we say the test is inconsistent with our hypothesis.