Collecting Data

This lesson covers:

  1. What the terms population, sample, sample size, representative, and bias mean
  2. What random sampling is
  3. The different methods of collecting data, including questionnaires, interviews, and observations

Who to collect data from


You can collect data from either the entire population or just a sample of the population.

Population - every single individual in the group that you are interested in. For example, every student at your school.

Sample - a subset of the whole group that you collect data from. For example, the 20 students that were easy to find in the playground.


When collecting data from a sample you need to consider:


Sample size - the number of individuals in the sample. The larger the sample size, the more accurately it will represent the population.

Whether it is representative - this refers to whether or not your sample accurately represents the whole population it came from.

Whether it is biased - biased data is data which does not represent the whole population. It can be avoided by choosing the right sampling method or increasing the sample size.

What does 'sample size' mean? 

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What does it mean for a sample to be representative? 

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What does it mean for a sample to be biased? 


Give 2 ways it can be avoided.

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Random sampling


To make a sample as representative as possible (and avoid bias), we usually use random sampling.


This is where every member of a population has an equal chance of being in the sample. 

How to choose a random sample:


  1. Assign a number to every member of the population. 
  2. Use a random number generator to create a list of random numbers. 
  3. Match the list of random numbers to members of your population and only include those individuals in your sample.

Does using random sampling increase or decrease bias?

Increase bias

Decrease bias

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How to collect the data


There are many different methods for collecting data. Three examples are:


  1. Questionnaires (including online surveys) - these are usually lists of questions about a topic. 
  2. Interviews - a structured conversation where one person asks questions, and the other provides answers.
  3. Observations - a way of gathering data by watching behaviour or events and recording anything useful.