Ttest

Hello

Can someone explain what this function does? I have to do a support call
for a customer and have never come across the Ttest function before. The
answer that I have in the cell containing the function has got E-15 after it
(which I don't understand either). I've also been asked how to calculate
probability. The version is 2000 - I can supply more detail if the above
isn't adequate.

Many thanks for your help.

Anita

Anita -

Search Excel's Help for "ttest" and for "scientific notation" or "scientific
format."

For more information about t-test, consult a statistics textbook.

- Mike
www.mikemiddleton.com

"Anita" <Anita@discussions.microsoft.com> wrote in message
news:AC6318D8-27DA-4505-8C52-8B682D2082CF@microsoft.com...
> Hello
> Can someone explain what this function does? I have to do a support call
> for a customer and have never come across the Ttest function before. The
> answer that I have in the cell containing the function has got E-15 after
> it
> (which I don't understand either). I've also been asked how to calculate
> probability. The version is 2000 - I can supply more detail if the above
> isn't adequate.
> Many thanks for your help.
> Anita

Anita

The TTEST function is a statistical test known as the t-test.

Put simply, it compares the means of two samples to assess whether they
differ significantly or not. The output is a probability value. In general a
value less than 0.05 would indicate that the two means are different.

For example, suppose you had two classes of schoolchildren, each of class
size 30. Suppose they all took the same test and received a mark out of 100.
A question may be to test whether the average (mean) mark for one class is
significantly different than the other class.

Class A ClassB
87 82
92 77
63 75
70 73
65 87
etc

In Excel you may have column A1 as class A results and column B1 as class B
results.

The mean of class A maybe 80 and the mean of class B maybe 75 ( I am making
these numbers up). So, does this suggest that the average results are
signifcantly different?

What you would do is...

TTEST([input range for classA],{input range for classB], 2,2)

And this will give a probability value e.g. 0.15. This means that the chance
of observing those results would occur about 15% by chance anyway. Typically,
a p-value of less than 0.05 (5%) is considered significant i.e. those results
would only be obtained by chance 5% of the time.

This is a very brief overview into a subject that has many nuances. This is
a light overview. If that satisfies your curiousity then fine...if not then
please do write back and I shall endeavour to explain further.

Regards

Alex



"Anita" wrote:

> Hello
>
> Can someone explain what this function does? I have to do a support call
> for a customer and have never come across the Ttest function before. The
> answer that I have in the cell containing the function has got E-15 after it
> (which I don't understand either). I've also been asked how to calculate
> probability. The version is 2000 - I can supply more detail if the above
> isn't adequate.
>
> Many thanks for your help.
>
> Anita

Yes the explanation of Ttest is very helpful thanks.

But, I still don't understand why I have an E-15 at the end of the answer.
If I change the number of decimal places to show 15 then I get a very small
number; does this therefore mean that the two means are different because the
value is significantly less than 0.05? The answer I get is something like
0.0000000000000234. Or do you think the formula is totally incorrect (I
haven't checked it at this stage)? Would you expect to get an answer like
this?

Thanks again

Anita

"Alex" wrote:

> Anita
>
> The TTEST function is a statistical test known as the t-test.
>
> Put simply, it compares the means of two samples to assess whether they
> differ significantly or not. The output is a probability value. In general a
> value less than 0.05 would indicate that the two means are different.
>
> For example, suppose you had two classes of schoolchildren, each of class
> size 30. Suppose they all took the same test and received a mark out of 100.
> A question may be to test whether the average (mean) mark for one class is
> significantly different than the other class.
>
> Class A ClassB
> 87 82
> 92 77
> 63 75
> 70 73
> 65 87
> etc
>
> In Excel you may have column A1 as class A results and column B1 as class B
> results.
>
> The mean of class A maybe 80 and the mean of class B maybe 75 ( I am making
> these numbers up). So, does this suggest that the average results are
> signifcantly different?
>
> What you would do is...
>
> TTEST([input range for classA],{input range for classB], 2,2)
>
> And this will give a probability value e.g. 0.15. This means that the chance
> of observing those results would occur about 15% by chance anyway. Typically,
> a p-value of less than 0.05 (5%) is considered significant i.e. those results
> would only be obtained by chance 5% of the time.
>
> This is a very brief overview into a subject that has many nuances. This is
> a light overview. If that satisfies your curiousity then fine...if not then
> please do write back and I shall endeavour to explain further.
>
> Regards
>
> Alex
>
>
>
> "Anita" wrote:
>
> > Hello
> >
> > Can someone explain what this function does? I have to do a support call
> > for a customer and have never come across the Ttest function before. The
> > answer that I have in the cell containing the function has got E-15 after it
> > (which I don't understand either). I've also been asked how to calculate
> > probability. The version is 2000 - I can supply more detail if the above
> > isn't adequate.
> >
> > Many thanks for your help.
> >
> > Anita

Anita

The E-15 is just a shorthand way of writing the number 0.0000000000000234.
So they are the same. A result like this can be achieved and so I 'presume'
the formula is correct.

Yes, your value is less than 0.05 and therefore is statistically significant
on the face of it.

To give more detailed answers I would have to know the nature of the inputs
to the TTEST and what hypothesis is being tested.

As an aside, the thing most likely to invalidate the interpretation of a
TTEST is not the value that the TTEST function returns, but the actual nature
of the data and the hypothesis being tested. So I wouldn't worry too much
about the TTEST result...rather I would pay closer attention to the nature of
the data being analysed...in order for a TTEST to be valid certain 'criteria'
must be met otherwise the test maybe inappropriate...

Hope this helps...

Alex

"Anita" wrote:

> Yes the explanation of Ttest is very helpful thanks.
>
> But, I still don't understand why I have an E-15 at the end of the answer.
> If I change the number of decimal places to show 15 then I get a very small
> number; does this therefore mean that the two means are different because the
> value is significantly less than 0.05? The answer I get is something like
> 0.0000000000000234. Or do you think the formula is totally incorrect (I
> haven't checked it at this stage)? Would you expect to get an answer like
> this?
>
> Thanks again
>
> Anita
>
> "Alex" wrote:
>
> > Anita
> >
> > The TTEST function is a statistical test known as the t-test.
> >
> > Put simply, it compares the means of two samples to assess whether they
> > differ significantly or not. The output is a probability value. In general a
> > value less than 0.05 would indicate that the two means are different.
> >
> > For example, suppose you had two classes of schoolchildren, each of class
> > size 30. Suppose they all took the same test and received a mark out of 100.
> > A question may be to test whether the average (mean) mark for one class is
> > significantly different than the other class.
> >
> > Class A ClassB
> > 87 82
> > 92 77
> > 63 75
> > 70 73
> > 65 87
> > etc
> >
> > In Excel you may have column A1 as class A results and column B1 as class B
> > results.
> >
> > The mean of class A maybe 80 and the mean of class B maybe 75 ( I am making
> > these numbers up). So, does this suggest that the average results are
> > signifcantly different?
> >
> > What you would do is...
> >
> > TTEST([input range for classA],{input range for classB], 2,2)
> >
> > And this will give a probability value e.g. 0.15. This means that the chance
> > of observing those results would occur about 15% by chance anyway. Typically,
> > a p-value of less than 0.05 (5%) is considered significant i.e. those results
> > would only be obtained by chance 5% of the time.
> >
> > This is a very brief overview into a subject that has many nuances. This is
> > a light overview. If that satisfies your curiousity then fine...if not then
> > please do write back and I shall endeavour to explain further.
> >
> > Regards
> >
> > Alex
> >
> >
> >
> > "Anita" wrote:
> >
> > > Hello
> > >
> > > Can someone explain what this function does? I have to do a support call
> > > for a customer and have never come across the Ttest function before. The
> > > answer that I have in the cell containing the function has got E-15 after it
> > > (which I don't understand either). I've also been asked how to calculate
> > > probability. The version is 2000 - I can supply more detail if the above
> > > isn't adequate.
> > >
> > > Many thanks for your help.
> > >
> > > Anita

On Fri, 17 Jun 2005 04:53:07 -0700, "Anita"
<Anita@discussions.microsoft.com> wrote:

>Yes the explanation of Ttest is very helpful thanks.
>
>But, I still don't understand why I have an E-15 at the end of the answer.
>If I change the number of decimal places to show 15 then I get a very small
>number; does this therefore mean that the two means are different because the
>value is significantly less than 0.05?

Assuming you have selected the ranges correctly, that's exactly what
it means.

(0.05 is not the only standard of comparison, but it's the most
common. Assuming the two data sets are random samples, the E-15 days
there's only about one chance in a thousand million million that the
difference in the data sets could have come about by chance. The
calculation of probability takes the sizes, averages, and scatter of
the data sets into account.

--
Stan Brown, Oak Road Systems, Tompkins County, New York, USA
http://OakRoadSystems.com
A: Maybe because some people are too annoyed by top-posting.
Q: Why do I not get an answer to my question(s)?
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?