# Statistics

Viewing 21 posts - 1 through 21 (of 21 total)
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• #38679

guglani
Member

Hi!I  want to know about the test to compare two percentages from the same sample.

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#116111

Joseph Banerjee
Participant

you want to compare the mean percentage of the two samples if yes then depending on the sample size you can go for one sample z or t test again depending if your data is normal.

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#116112

Mikel
Member

Bad call. It is a test of two proportions found under stat – basic stat in Minitab.
Proportions follow a binomial distribution. If certain assumptions are met, the assumption of normality may hold, but why bother with that – do it right.

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#116113

Ken Feldman
Participant

WRONG!!!!!  You use a 2 proportion test.  You use a one sample z to compare one sample to a standard…continuous data and a t test for smaller samples.  A two sample t test for comparing the continuous mean of two samples.  The 2 proportion test is for two proportions which is the same as two percentages which is what the poster asked about.

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#116115

Ken Feldman
Participant

Stan, u r up early today.  Beat me to the answer to this wrong response.  Saw that Silicon Valley Stan finally gave his notice.  A teary eyed farewell note to the gang….sob sob…  Have a good day.

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#116117

indresh
Participant

have a query might be trivial
percentage is a discrete data then why use T-test
can’t we use chi sqaure to know if there is commonality between the standard and the sample in question ????
rgds,

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#116119

Joseph Banerjee
Participant

Stan its not necessary that proportions to follow a binomial distribution

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#116121

Joseph Banerjee
Participant

Hi Indresh you need to ask yourself whether percentage is a discrete data.

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#116122

DaveS
Participant

Stan-
Usually agree with you, but this time you’re aoff base. the “proportion test” is a Z test , plain and simple.
So your comment “If certain assumptions are met, the assumption of normality may hold, but why bother with that – do it right.” makes no sense. What would be doing it riight? Please provide an example. If the binomial approximations are not met  the alpha risk is increased, but there is no parametric “proportions” test other than using the Z test.
Reference ” Handbook of Parametric and Nonparametric Statistical Procedures, Sheskin.

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#116125

Mikel
Member

Wrong

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#116126

Mikel
Member

Wrong

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#116127

Mikel
Member

Joseph,
Go learn something before you give advice

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#116129

DaveS
Participant

That’s right!
You’re wrong!

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#116131

Fer
Participant

Why should a proportion test be a Z-test? You use the binomial distribution to compute the confidence intervals

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#116132

Mikel
Member

Daves,
Before you get too far down this trail, go to the help menu in Minitab on the test of two proportions under stat – basic stat. Read.
Let’s assume you are right. I have one proportion – I have one defect in a sample of 30. I have another proportion I have 5 defects in a sample of 100. Is there a statistical difference and how exactly do you perform a z test on this?
Humble (or not so humble) apologies will be accepted.

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#116134

DaveS
Participant

Staney boy-
Please read from MINITAB help, Formulas and methods. I’m not apologizing for squat. It is simply an application of the Z test. It uses the binomial mean and sd, but the test statistic is Z.

Hypotheses

The null hypothesis for a 2 proportion test is: H0: p1 – p2 = d0
where:
·    p1 = the proportion for the first population
·    p2 = the proportion for the second population
·    d0 = the hypothesized difference between the population proportions
You can choose any one of three alternative hypotheses:

H1: p1 – p2 > d0

One-tailed test

H1: p1 – p2 < d0

One-tailed test

H1: p1 – p2    ddd0

Two-tailed test

Test statistics

The calculation of the test statistic, Z, depends on the method used to estimate p.
By default, Minitab uses separate estimates of p for each population and calculates Z by:

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#116136

Mikel
Member

Now that you have given enough information that the person can doing something with (if they choose to do by hand), I agree.
My original response was correct of where to go and what test to run. If the original poster simply goes to a z test in Minitab, they wouldn’t know what to do.
Peace

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#116137

Mikel
Member

Also note that you get the following warning from Minitab when doing the sample I suggested –
* NOTE * The normal approximation may be inaccurate for small samples.
This is because we are using the normal (z) as an approximation of the binomial and there are certain assumpltions that must be met. The assumprions are simple np and n(1-p) must both be greater than 5. There is a reason I gave you the example I did – I knew it did not meet the assumptions.
The correct way to do this is to get the confidence intervals of the two proportions using binomial and see if proportion 1 is inside of the confidence interval for proportion 2 and vice versa.

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#116139

Chwang
Participant

Average men working in earnest together complete the building of the temple before the wisest solitary man will place even one stone.

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#116166

Mikel
Member

Hey Confucius,
Strange to see you show up on a site dedicated to rampant profitering. Last time I checked, you were against such things.
Be that as it may, let me give you my new favorite Confucius type sayings –
1) Be careful when trying to sound like Confucius, people will not know if you are an idiot or a wise man who just sounds like an idiot.
2) Average men, with a little learning that they think is great learning, working in earnest together, are just a group of average men with a little learning.

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#116289

Chwang
Participant

A learned man who casts away all who approach him is valued by men as is a large rock atop the highest mountain.

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