DAT Math Standard Deviation

[moose786]

So what's the correct formula for std. dev. & variance?

Chad uses "N-1" in his st. dev & variance formulas, whereas Destroyer & Bootcamp use "N"?

Tried searching old forums and there were mixed answers.

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[TimeTraveler]

I was confused about this my entire time studying for the test. By the last week of studying, I just told myself I'm going to use N, since I got it wrong on every practice test using N-1.

 

[FutureDDS_11]

I think understanding it will help you clear up the confusiom:
N is used to get the sample standard deviation. N-1 is used to estimate to standard deviation of the population. This will help you remember it: the reason n-1 is used is because you want a larger value to get a less fixed estimate (because each sample in a population can be slightly different), so you divide by a smaller number to have a slightly bigger variance and therefore get a slightly more accurate estimate of the population variance.
Most of the time for the DAT you're given a set of numbers, which is a sample, so you use N. In case they ask to estimate the population variance/std dev., then you would use n-1

 

[moose786]

I think understanding it will help you clear up the confusiom:
N is used to get the sample standard deviation. N-1 is used to estimate to standard deviation of the population. This will help you remember it: the reason n-1 is used is because you want a larger value to get a less fixed estimate (because each sample in a population can be slightly different), so you divide by a smaller number to have a slightly bigger variance and therefore get a slightly more accurate estimate of the population variance.
Most of the time for the DAT you're given a set of numbers, which is a sample, so you use N. In case they ask to estimate the population variance/std dev., then you would use n-1

I remember reading an answer similar to yours in another forum, but either I just had a blast from the past on what I learned from the greatest AP Stats teacher of all time (took 3.5 years ago) or I just had a brain fart that time I read the other forum. Thank you for your wording, sir. I remember more clearly having to do this in statistics.

Do you have any recommendations on good videos for probability? Khan Academy has a huuuuge playlist on it that includes permutations & combinations (my weaknesses right now since I forgot how I used to so easily do them back in high school)...but that's just my problem. I don't want to lose valuable time trying to sift through videos whose content I already mostly know, and also risk not finding the content I'm looking for either. That, or maybe different scenarios/ways that you signal to yourself on how to tackle the problem? Key words? Anyone and everyone is open to answer this for me, haha. Please. Much obliged.

 

[Roy Williams]

I remember reading an answer similar to yours in another forum, but either I just had a blast from the past on what I learned from the greatest AP Stats teacher of all time (took 3.5 years ago) or I just had a brain fart that time I read the other forum. Thank you for your wording, sir. I remember more clearly having to do this in statistics.

Do you have any recommendations on good videos for probability? Khan Academy has a huuuuge playlist on it that includes permutations & combinations (my weaknesses right now since I forgot how I used to so easily do them back in high school)...but that's just my problem. I don't want to lose valuable time trying to sift through videos whose content I already mostly know, and also risk not finding the content I'm looking for either. That, or maybe different scenarios/ways that you signal to yourself on how to tackle the problem? Key words? Anyone and everyone is open to answer this for me, haha. Please. Much obliged.

What area do you want to focus on for probability? It's a huge field. Mainly want to stick to permutations and combinations or are there other subjects too?

 

[moose786]

What area do you want to focus on for probability? It's a huge field. Mainly want to stick to permutations and combinations or are there other subjects too?

Mainly those 2 for now. I understand how they work and I've seen videos and can follow along, but I for some reason, always have trouble realizing what to do next in certain problems. Actually, let me rephrase. I go about it a certain way, thinking I'm doing it right and there's usually SOMEthing I messed up out of misinterpretation. And this happens for a lot of the combinations/permutations/probabilities involving combinations & permutations. I've done Chad's videos on this, but still am bad at grasping.

Also, I want to focus on those "at least, at most, and, or" probabilities. I'm still at the relatively early stages in my Math studying, but I find Math Destroyer's explanations to be lacking for things that I forgot about since high school. Which is understandable. It would take maybe twice the book's size to explain each problem in-depth like that. And also, the book is for practice, not for teaching things from scratch.

Got a 19 on my Bootcamp Exam 1 attempt last night for the first time. I don't know what that says exactly, but I did enjoy doing SAT Math back in the day (even though I could never perfect it). After reviewing my mistakes, I saw there were like 4 problems out of the 9 (I think it was 9) that I got wrong because of the whole permutations/combinations/probabilities stuff I suck at. And also, 3 that were completely stupid errors.

The 19 kinda gave me a confidence boost since I've barely done math review yet, and have yet to memorize formulas and FIGURE OUT THESE DAMN perm/comb/probability problems. Once I have that, as well as a decent amount of Math Destroyer mastered, I'll tackle the next exam...

Kinda ranted, but I hope that answered your question, sir.

 

[Roy Williams]

Mainly those 2 for now. I understand how they work and I've seen videos and can follow along, but I for some reason, always have trouble realizing what to do next in certain problems. Actually, let me rephrase. I go about it a certain way, thinking I'm doing it right and there's usually SOMEthing I messed up out of misinterpretation. And this happens for a lot of the combinations/permutations/probabilities involving combinations & permutations. I've done Chad's videos on this, but still am bad at grasping.

Also, I want to focus on those "at least, at most, and, or" probabilities. I'm still at the relatively early stages in my Math studying, but I find Math Destroyer's explanations to be lacking for things that I forgot about since high school. Which is understandable. It would take maybe twice the book's size to explain each problem in-depth like that. And also, the book is for practice, not for teaching things from scratch.

Got a 19 on my Bootcamp Exam 1 attempt last night for the first time. I don't know what that says exactly, but I did enjoy doing SAT Math back in the day (even though I could never perfect it). After reviewing my mistakes, I saw there were like 4 problems out of the 9 (I think it was 9) that I got wrong because of the whole permutations/combinations/probabilities stuff I suck at. And also, 3 that were completely stupid errors.

The 19 kinda gave me a confidence boost since I've barely done math review yet, and have yet to memorize formulas and FIGURE OUT THESE DAMN perm/comb/probability problems. Once I have that, as well as a decent amount of Math Destroyer mastered, I'll tackle the next exam...

Kinda ranted, but I hope that answered your question, sir.

PM'ing you