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## Statistics and probability

### Course: Statistics and probability>Unit 6

Lesson 4: Types of studies (experimental vs. observational)

# Appropriate statistical study example

Sal determines if a statistical study was a sample study, an experiment, or an observational study. Created by Sal Khan.

## Want to join the conversation?

• I feel like this question is set up to trick the user. In the first paragraph, Alma does an EXPERIMENT (but does not perform the experiment properly since there is no control group). Then in the second paragraph she takes a SAMPLE of the petri dish. So there are actually two types of study being used.

Then Alma samples 300 bacteria from the dish, but how do we know how many were in the dish to begin with? I would guess billions of bacteria, but there's no indication of how many there were in total.

Very confusing
• You're right. It WAS an experiment as well as sample study. Sal should correct this video. Experiment is a broad term and there are many different types of experiments. The experiments he was talking about are only one specific type of experiment. A good reliable type but not the only kind.
• It is the apparent paradox you get when comparing two groups and a trend you see between the two groups over a set of samples disappears or often reverses when all the data is combined.

For example, if you look at batting averages (example borrowed from wikipedia) you see that although David Justice had a higher batting average than Derek Jeter in both 1995 and 1996, Jeter actually has the higher average over the combined two years.
...........................1995............1996...............Combined
Derek Jeter 12/48 .250 183/582 .314 195/630 .310
David Justice 104/411 .253 45/140 .321 149/551 .270
• This is an explanation of one of the answers to the questions in types of
statistical studies:
"Even though experiments suggest causation, it would be too far-reaching to conclude that "meditation reduces work stress," which implies that meditation would reduce work stress for anyone who practiced it"
I don't understand why this conclusion is not allowed
• I'm not really sure why your answer was wrong. If it was a randomized experiment, then concluding causality is generally acceptable. My only thought is that perhaps one of the other answers was slightly different.

For some comparison, I went through a number of the questions, and at times for a "randomized experiment" type of question there were two answers that were very similar, but one was a bit more general in terms of how they phrased the results. For instance, some of the questions were about "7-11 year old children" eating more food when watching TV commercials featuring snacks. Two of the answers had the proper conclusion, but one of them phrased it in terms of 7-11 year old children, while the other phrased it more broadly (I think it was all children, or all people).

In your comment you didn't fully clarify the question and answers. Could it be that the question described a more specific population (e.g., adults, or adult men, etc.)? The conclusion of our study can only apply to the same population from which the sample was drawn.

Without seeing the exact text of the question and answers (I looked a bit through the questions for it), I'm not sure that I could provide a better answer.
• What's the difference between a sample study and an observational study ?
(1 vote)
• in which exposure and outcome are determined simultaneously for each subject.
(1 vote)
• This is some explanation from the questions in types of statistical studies:
"Even though experiments suggest causation, it would be too far-reaching to conclude that "meditation reduces work stress," which implies that meditation would reduce work stress for anyone who practiced it."
bold Why can't you that "meditation reduces work stress"
(1 vote)
• Couldn't we describe it as a faulty experiment ? Sal's seemed to do so , when he was answering the second question
(1 vote)
• If she is finding a correlation between the effectiveness of the bacteria and the death percentage isn't this study an observational study?
(1 vote)
• Hi!

In what cases should an observational study or a sample study be performed instead of an experiment?
(1 vote)
• 1. ethical reasons
some studies (many in medical senses) cannot be allowed to perform to human subjects. thus observational study could and should be the next best option. but in some cases, there aren't enough observational data yet. then you could do sample studies to get enough data for each of variables you're curious of if which have a correlation. and then move into the observational study

2. physical impossibility
if you were interested in the relationship between the aging rate of stars in the universe and the aging rate of living organisms on the planets around them, you better rely on both types of study (observational and sample). because first, you can't experiment with the whole universe due to its sheer scale. second, you can't control the aging of the stars or the organisms in either way. and the third reason for this study to have to rely on some type of sampling is same as above, you may have not enough datapoints (especially for the aging rate of living organism part cause we have one and only data on this very earth yet)

in short, some experiments can be performed but won't be for the ethical reason. while a few cannot be performed physically in the first place

q. i know the aging rate example sounds so unrealistic that gives you some perception that most realistic experiments could be performed in a physical manner. if you do feel this way, what would be a realistic case in which we literally can't perform an experiment thus have to rely on the other options like observations and samplings?
(1 vote)
• In this case, a more appropriate experiment would also include a control group on a accepted antibiotic...
(1 vote)
• You can determine a sample study by someone's confidence level. You can determine a experiment by measuring a centrality, such as the mean, median, or proportion; and a measure of variability, such as the standard deviation. You can determine observational study by having researches observe the effect of a risk factor, diagnostic test, treatment or other intervention without trying to change who is or isn't exposed to it.
(1 vote)