Normal Distribution....

[purplelife]

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Can someone please explain me this question and how to go about it quickly?

It looks so simple, but I want to make sure I'm doing it correctly. I have been looking around at the videos on YouTube and other places that explains similar question, but they all related to charts which I don't think we are supposed to use on step 1.

The question is:

mean=220 mg/L
Sd=10
n= study of 400
How many pts will have cholesterol > or equal to 240mg/dL?

Thank you!!

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The answer choices are:
a- 2
b- 10
c-20
d-64
e-128

answer: b-10

 

[mmmcdowe]

So basically, how many people are two standard deviations or more to the right. So subtract 100% from the area within 2 SD, then divide by two (because you only want one side, not both). That will give you the percent of N that you have.

 

[Ycut]

Can someone please explain me this question and how to go about it quickly?

It looks so simple, but I want to make sure I'm doing it correctly. I have been looking around at the videos on YouTube and other places that explains similar question, but they all related to charts which I don't think we are supposed to use on step 1.

The question is:

mean=220 mg/L
Sd=10
n= study of 400
How many pts will have cholesterol > or equal to 240mg/dL?

Thank you!!

2 SD above the mean 16% = 64 pts

what are the options ?

 

[Textbookversion]

2 SD above the mean 16% = 64 pts

what are the options ?

No. Just no. It's 2 SD = 5%/2 = 2.5% = 10 patients. (It's actually 9.1 patients if you use the non rounded value).

 

[Suncrusher]

Can someone please explain me this question and how to go about it quickly?

It looks so simple, but I want to make sure I'm doing it correctly. I have been looking around at the videos on YouTube and other places that explains similar question, but they all related to charts which I don't think we are supposed to use on step 1.

I don't know what charts you are talking about, but there is a biostats knowledge base you are supposed to memorize for step1, and this is part of it. This question is simple and quick IF you've memorized that, assuming a normal distribution:

* 68% of the patients will fall within 1 sd (standard deviation) of the mean (34% within 1sd above the mean, 34% within 1sd below the mean)
* 95.5% of the patients will fall within 2 sd of the mean
* 99.7% of the patients will fall within 3 sd of the mean.

You simply should memorize that 68% goes with 1sd, 95.5% goes with 2sd, and 99.7 goes with 3sd. If you have that basic biostats knowledge base, you can then more easily figure out that because the mean is 220 and the sd is 10, the question is asking for how many patients were 2sd ABOVE (above only, not below) the mean.

Since you've memorized that 95.5% of patients fall within 2sd of the mean, you can calculate that 382 patients (0.955*400) are between the range of 200-240. Or you can skip straight to calculating that 18 patients (.045*400=18, since 100%-95.5%=4.5%) fall outside 2sd of the mean, half below 200 and half above 240. Since you only want the ones above 240, the answer is half of those 18 patients, or 9.

 

[Ycut]

No. Just no. It's 2 SD = 5%/2 = 2.5% = 10 patients. (It's actually 9.1 patients if you use the non rounded value).

why aren't u counting percentage of 2sd

 

[MLT2MT2DO]

why aren't u counting percentage of 2sd

He is. 5% of patients will fall outside of 2SD. 2.5% of that 5% will be <200 and 2.5% will be >240.

 

[purplelife]

I don't know what charts you are talking about, but there is a biostats knowledge base you are supposed to memorize for step1, and this is part of it. This question is simple and quick IF you've memorized that, assuming a normal distribution:

* 68% of the patients will fall within 1 sd (standard deviation) of the mean (34% within 1sd above the mean, 34% within 1sd below the mean)
* 95.5% of the patients will fall within 2 sd of the mean
* 99.7% of the patients will fall within 3 sd of the mean.

You simply should memorize that 68% goes with 1sd, 95.5% goes with 2sd, and 99.7 goes with 3sd. If you have that basic biostats knowledge base, you can then more easily figure out that because the mean is 220 and the sd is 10, the question is asking for how many patients were 2sd ABOVE (above only, not below) the mean.

Since you've memorized that 95.5% of patients fall within 2sd of the mean, you can calculate that 382 patients (0.955*400) are between the range of 200-240. Or you can skip straight to calculating that 18 patients (.045*400=18, since 100%-95.5%=4.5%) fall outside 2sd of the mean, half below 200 and half above 240. Since you only want the ones above 240, the answer is half of those 18 patients, or 9.

Thanks everyone for your input.. It kinda makes sense to me now.

 

[CDI]

2 SD above the mean 16% = 64 pts

what are the options ?

Rules in Normal Distribution:
68% of people always fall within 1 SD below or above the mean.
(So here, the middle 68% get between 210-230)

95% always fall within 2 SD below or above the mean.
(So here, the middle 95% get between 200 - 240)

Then out of the 5% remaining, it's evenly split. So, 2.5% are in the bottom end (below 200), and 2.5% are in the top end (above 240).

So, the question boils down to:
How many are in the top 2.5% (above 240) of the 400 people?

10% of 400 = 40
5% of 400 = 20
2.5% of 400 = 10

 

[Ycut]

Rules in Normal Distribution:
68% of people always fall within 1 SD below or above the mean.
(So here, the middle 68% get between 210-230)

95% always fall within 2 SD below or above the mean.
(So here, the middle 95% get between 200 - 240)

Then out of the 5% remaining, it's evenly split. So, 2.5% are in the bottom end (below 200), and 2.5% are in the top end (above 240).

So, the question boils down to:
How many are in the top 2.5% (above 240) of the 400 people?

10% of 400 = 40
5% of 400 = 20
2.5% of 400 = 10

i understand that cholesterol level >240 is above 2SD

but what about cholesterol = 240
is not it in 2SD

 

[Suncrusher]

so since OP edited to include the answer choices:

10 was in the answer choices but 9 wasnt. Is step1 apparently simplifying the statistics (i.e. we are supposed to use 95% for our calculations instead of the true 95.5%)? I guess there will probably only ever be one answer choice that is close enough, so it wont matter which one people use...

edit: nevermind I just looked it up and 95.5ish isnt even completely true, it's rounded off from an even more accurate number slightly lower than itself...which is why some people round all the way down to 95 instead of up to 96... I guess it just depends how much you want to round. no biggie.

 

[CDI]

i understand that cholesterol level >240 is above 2SD

but what about cholesterol = 240
is not it in 2SD

Its not an engineering exam. They just want to see that you're not completely missing the concept being asked. Anyone who's done UW Qs or been through Step 1 knows the answer choices for statistics problems are always far apart so one is a clear cut choice (even if not completely accurate).

 

[purplelife]

It would be really nice if someone made a youtube video on few of these questions because it still confuses me and takes up a lot of my time during test. It would help other students too. I'm studying at home so I'm on my own for now and use SDN for any questions I have. All my classmates/friends are studying too and are super busy to take the time to respond.

 

[withrye]

It would be really nice if someone made a youtube video on few of these questions because it still confuses me and takes up a lot of my time during test. It would help other students too. I'm studying at home so I'm on my own for now and use SDN for any questions I have. All my classmates/friends are studying too and are super busy to take the time to respond.

Maybe it'll help to think graphically. You can see on the bell curve below what percentage of the curve lies in which areas. Questions are going to generally just ask you about what percentage lies inside or outside of 1SD, 2SD, or 3SD.

For the "inside xSD" question, just add up the percentages that live between those points on the curve. For 1SD, adding up all the numbers between -1 and 1 brings you to 68.2%. For 2SD, the summation will bring you to 95.4%. For 3SD, it's 99.8%.

Your original question was a "greater than xSD question." Looking at the graph and adding up all the percentages above 2SD, you end up with 2.3%.* So you know that around 2.3% of the population in question will be over 2SD away from the mean metric.

http://www.regentsprep.org/Regents/math/algtrig/ATS2/normal67.gif

*For the sake of mental math, we round down to 68% (inside 1SD), down to 95% (inside 2SD), and therefore up to 2.5% to compensate (north of 2SD).

 

[Jonari]

It would be really nice if someone made a youtube video on few of these questions because it still confuses me and takes up a lot of my time during test. It would help other students too. I'm studying at home so I'm on my own for now and use SDN for any questions I have. All my classmates/friends are studying too and are super busy to take the time to respond.

Seriously. The Kaplan behavioral science standard deviation is a bit too easy, in my opinion, and not as challenging. I definitely don't want to miss points on my exam, because of the biostats questions. 😳

 

[CDI]

Seriously. The Kaplan behavioral science standard deviation is a bit too easy, in my opinion, and not as challenging. I definitely don't want to miss points on my exam, because of the biostats questions. 😳

Yeah...I had more than my fair share. I was doing math half the time, and interpreting graphs which ppl hardly even use anymore.

 

[purplelife]

I'm really grateful to all of you for your replies... Thank you very much for taking out the time to reply to others studying for the steps.

You guys are awesome!