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## Statistics and probability

### Course: Statistics and probability>Unit 6

Lesson 3: Sampling methods

# Techniques for random sampling and avoiding bias

Techniques for random sampling and avoiding bias.

## Want to join the conversation?

• what is the difference between a cluster sample and a stratified random sample?
• In clustered, there are many groups with equal balanceof level/gender. In stratified, There are many groups. Each group is completely one level/gender. other than that it is similar.
• Is it possible that clustering technique itself can introduce bias? Sal's example of sampling by classroom might allow selection of an even male/female sample but isn't this a bit risky? Factors that affect outcome (maybe more strongly than gender) may cluster in classrooms - e.g. teacher quality, classroom resources, social groups, or some unknown factor(s). This may return us to another problem of random/fair sampling from among clusters
• When would you use non-random sampling?
• to be more realistic and somehow bittersweet, when you want to win an election or competition, and simply to show off yourself to others

i mean a statistical massage is in your hands with non-random sampling
(1 vote)
• Sal mentions that in a stratified sample he could take 25 students from each year to make up the 100 student sample. But what if there are say 50% more seniors than juniors. Wouldn't you have to take more from the seniors in order to reduce the bias?
• to
Can't you just instead split your age group sample into genders too?
• That's definitely a possibility.

Cluster surveys are quick and effective, though.
Instead of tracking down people one by one, upon which half of them will probably say that they don't have time to answer,
you just go into the classroom and wait for five minutes, and because they are in a group it's much easier to get a response from everyone.
• Doesn't the clustered system introduce a lot of bias?

For instance, in the example in the video they seem to choose a single class in each of the 4 years.
- within students of a single class, there's a lot more shared history than between randomly chosen students. So that will probably influence the results.
- if one of the chosen classes is significantly smaller or bigger than one of the other chosesn classes, that year will be over- or under-representated ...
• When do you use stratified sampling vs clustered sampling besides cluster sampling being more for geographical purposes?
• Doesn't the stratified method also introduce some bias?

For instance, the method used in the example in this video assumes that the school has an equal number of students in each of the 4 years ...