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### Course: AP®︎/College Statistics>Unit 6

Lesson 4: Introduction to experimental design

# Introduction to experiment design

Explanatory and response variables. Random sampling and random assignment (including block design). Placebos and blind/double-blind experiments.

## Want to join the conversation?

• At , Sal mentions that you can use "block design" to randomly divide patients with common properties, such as sex, evenly throughout the control and experiment groups. Isn't that the same as using stratified sampling in a sample study?
• I think it is the same idea, it just has a different name. Experimental surveys and observational studies are pretty different in form, so I think it would be confusing to use the same term for both.
Hope this helps!
• how to control error using blocking
• You use blocking to minimize the potential variables(also known as extraneous variables) from influencing your experimental result.

Let's use the experiment example that Mr.Khan used in the video. To verify the effect of the pill, we need to make sure that the person's gender, health, or other personal traits don't affect the result. We want to test if the pill would be effective for everyone in general. This is where blocking method comes in; we need to divide the samples into two groups in a way that each group is similar to each other in terms of aforementioned factors(gender, etc.). From this, you can ensure that the result will be affected minimum by the unexpected factors, hence controlling the error.
• Is there a reason the KA video about correlation of A1C with blood sugar can't be found on this site anymore?
(It is still present on YouTube though)
• How could you confirm your experiment is replicable?, do you have to do another experiment with different populations or just mention on the potential paper that it is necessary to do more experiments considering other samples?
• it is usual that other researchers who have no direct interests in and are independent from one study would do the similar experiments, following the procedures and parameters published in the original paper.

then they check if their results would be compatible with the proposed ones.

and as you said, it's almost impossible to do exactly the same experiments with the same subjects again, espeically when it comes to the clinical cases like this. thus the purpose of replicating an experiment is not in duplicating the experiment and getting the exact same data itself, but in confirming general trends with similar conditions not by the authors of the paper but by the others (usually peer researchers).
• At I start getting confused
• What is the difference between block design and stratified sampling?
• Block design is when you have a sample and you, out of chance, have two or more categories that the individuals belong to, and then you have to divide those evenly into the control and treatment groups. Stratified sampling is when you, ahead of time, purposefully pick a fixed number from each category.
Mainly, the difference is that stratified sampling is on purpose, while block design happens because it was random.
(1 vote)
• whats the difference between block and strat
(1 vote)
• I came across a Khan Academy problem about an observational study showing a positive correlation between being a flight attendant and having cancer. According to the problem, whether or not someone is a flight attendant is the explanatory variable, and whether or not they have cancer is the response variable.

This is wrong, isn't it? It's not an experiment, so we can't assign these labels this way. I mean, for all we know, having cancer could cause someone to become a flight attendant.
(1 vote)
• how to control error using blocking
(1 vote)
• Is it really important to make a graph and if so why.
(1 vote)
• It's a REALLY good way to see what's going on with your data, and keep things in order and visually analyze your data. so Yes
(1 vote)