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Understanding how to choose a group of people that accurately represents a larger population.
If you wanted to know if a giant vat of soup was too salty, would you have to drink the entire thing, or would one well-stirred spoonful be enough?
In psychology, we rarely study every single person in a group. Instead, we define a Target Population, which is the entire group of individuals we want to understand (e.g., 'all teenagers in the US'). Because it is impossible to talk to millions of people, we select a Sample. The sample is the specific group of individuals that you will actually collect data from. For a study to be valid, the sample must be representative, meaning it looks like a 'mini-version' of the population. If your population is male and female, your sample should ideally reflect that same ratio.
Quick Check
If a researcher wants to study the sleep habits of all Grade 9 students in Canada but only surveys 200 students from one school, who is the 'Target Population'?
Answer
All Grade 9 students in Canada.
How do we ensure a sample is fair? We use Random Sampling. This means every member of the target population has an equal chance of being selected. Mathematically, if the population size is , every individual has a probability of of being chosen. This is like pulling names out of a giant, well-shaken hat. Random sampling helps eliminate selection bias, ensuring that the researcher’s personal preferences don't accidentally influence who gets picked.
1. Assign a unique number to every student in the Grade 9 directory ( to ). 2. Use a random number generator to pick numbers. 3. The students assigned to those numbers become your sample. 4. This ensures that a student's popularity or GPA doesn't affect their chance of being picked.
When a sample does not accurately represent the population, we have Sampling Bias. A common mistake in school research is Convenience Sampling—choosing people who are easiest to reach. For example, if you stand outside the gym to survey students about 'school spirit,' you might only get athletes. Another issue is Volunteer Bias, where people who offer to participate in a study might have stronger opinions or different personalities than those who don't.
A student researcher wants to know how much time the average student spends on social media. They decide to hand out surveys in the school library during lunch. 1. The Bias: Students in the library may be more likely to be studying than the general student body. 2. The Result: The data might show a 'social media use' average of hour, while the true population average is hours.
Quick Check
Why is 'Convenience Sampling' considered a threat to a study's validity?
Answer
It leads to a biased sample that doesn't represent the diverse characteristics of the whole population.
Sometimes, simple randomness isn't enough. If a school has Freshmen and Seniors, a purely random sample might accidentally pick only Seniors. To fix this, researchers use Stratified Sampling. They divide the population into sub-groups (strata) and then sample randomly from each group to maintain the correct proportions. If the population ratio is , the sample should also be .
You want a sample of students from a school where are girls and are boys. 1. Calculate the number of girls needed: . 2. Calculate the number of boys needed: . 3. Randomly select girls and boys separately. 4. This ensures your sample perfectly mirrors the gender balance of the school.
A researcher wants to study all high schoolers but only samples students from the honors chemistry class. This is an example of:
In a truly random sample, every individual in the target population has an equal chance of being chosen.
If a population has people and you need a sample using simple random sampling, how many people are in your sample?
Review Tomorrow
In 24 hours, try to explain the difference between a 'Target Population' and a 'Sample' to a friend without looking at your notes.
Practice Activity
Next time you see a survey result in the news, look for the 'Sample Size' and ask yourself: 'Who was left out of this group?'