How to Improve Validity of a Scientific Investigation

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  • 0:01 Validity in Science
  • 1:29 Improving Validity
  • 4:49 Lesson Summary
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Lesson Transcript
David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

Expert Contributor
Maria Airth

Maria has a Doctorate of Education and over 20 years of experience teaching psychology and math related courses at the university level.

This lesson explores one important measure of the quality of a scientific experiment: validity. Learn what validity is and how it can be improved in a scientific investigation.

Validity in Science

Although science is the best way to come up with accurate explanations for how the world works, not all scientific investigations are created equal. Some are better than others. There are a couple of ways of measuring how good a scientific investigation is. Two terms that are often used are reliability and validity. Reliability is a measure of how repeatable an experiment is - were the results are similar when the experiment was carried out multiple times. But perhaps more important than this is validity, which is a measure of how correct the results of an experiment are.

A particular experiment or investigation can be internally valid and externally valid. Internal validity is about whether the design of the experiment follows the standard steps of the scientific method, and whether the process followed by the experiment makes logical sense. External validity is about figuring out whether the conclusion from the experiment is the real explanation for the phenomenon in the wider world. It examines things like whether there might be an alternative explanation for the result.

If your goal is to make your result as close to the truth about the world as possible, then you need to improve your validity as much as you can. Most scientists are pretty successful making their experiments internally valid, but external validity can be harder to achieve. In this lesson, we going to take a look at a few ways you can improve the validity of your experiments.

Improving Validity

There are a number of ways of improving the validity of an experiment, including controlling more variables, improving measurement technique, increasing randomization to reduce sample bias, blinding the experiment, and adding control or placebo groups.

Controlling more variables is about making sure as few things as possible change during the experiment. An ideal experiment is where one thing is changed, and one result is looked at. Everything else should remain the same. So for example, if you wanted to know how fast balls of different masses will roll down a particular hill, you would change the mass of the ball, and keep everything else the same. You'd keep the material of the ball, the point of release, the measurement location a method, the humidity of the air, the height above sea level and anything else you can think of the same. The more you keep the same, the more likely it is that your result will be valid.

Measurement technique can also be improved. Perhaps instead of measuring something by hand, you could use a computer and electronic sensor. Or perhaps instead of having one person measure the results, you could have multiple people take measurements and compare their answers.

Increasing randomization is a way to reduce a particular validity problem: sample bias. That's when the samples being investigated are not a representative sample of the population. For example, say your testing for the effect of a drug and your trials contain mostly white males between 20 and 30 years old. That would not be a good sample due to the lack of breadth and age and gender. Or perhaps you are testing weight loss drugs on people who are of a healthy weight already. Increasing the randomization of the sample will reduce this problem.

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Additional Activities

Scientific Research Critique:

A great way to learn new concepts related to scientific experimentation is to examine those concepts in relation to published work. In this activity, students will review peer-reviewed, published scientific experiments to determine what methods were used to ensure validity and what could have been done differently to improve validity.


  • After reviewing the lesson, conduct a brief discussion with your students on the concepts learned in the lesson. Be sure to review the definitions of the main terms in the lesson:
    • Internal validity
    • External validity
    • Single-blind
    • Double-blind
    • Randomization
    • Controlling variables
  • When you are confident that your students have a strong understanding of these terms related to study validity, instruct them to select one scientific research article through research on databases designed for peer-reviewed scientific research or published journal articles.
    • Peer-reviewed articles are going to offer the best example to your students on how to correctly conduct research.
    • If you would like your students to have more opportunity for strong critiquing, allow them to choose research published in non-peer reviewed journals or magazines.
  • Now, instruct your students to focus on the validity of the study they have chosen. They should:
    • Review the article for evidence that each type of validity has been designed into the study.
    • Document every aspect of the study that adds to the strength of the validity of the whole study.
  • Finally, your students should write a brief summary of any suggestions they may have that would have increased the validity of the study in question. They should:
    • Indicate what type of validity their suggestion would impact.
    • Why the suggestion would increase the validity of the study.
    • Why they think the original study did not already include their suggestion.

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