Ttest

[Anita]

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

 

[Mike Middleton]

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

 

[Alex]

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]

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

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

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

 

[Alex]

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

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

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

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

 

[Stan Brown]

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?

 

[gbh]

Can you help me figure out if I'm interpreting the t-probability correctly?
I am comparing the means of 2 samples. With a 2-tailed test the probablility
statistic is 0.173 & with a 1-tailed it is 0.087. My interpretation is that
there is not a signigicant difference between the means using either the 1 or
2 tailed test. To be honest, I don't have a clue whether I should have used
a 1 or 2 tailed test so I did both. Can you also tell which I should be
using?
In another comparison of the means of 2 samples, the probability statistic
was 0.000346. I am interpreting this as a significant difference at the .03
CI. Is this correct?
gbh

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

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

 

[Jerry W. Lewis]

Most testing should be considered 2-tailed. The exception is if you can
honestly say yes to either of the following questions:

Before looking at the data, did you know which group MUST be larger if there
was a difference?

Before looking at the data, were you only interested in a difference if it
was in a particular direction (and it then turned out to be in that
direction)?

p=0.087 means that there is less than a 1 in 10 chance of observing such an
extreme difference (in the pre-specified direction) simply by chance if in
fact there is no difference. It is not significant at the 0.05 level, but
depending on your objectives, may be worth pursuing.

Jerry

Can you help me figure out if I'm interpreting the t-probability correctly?
I am comparing the means of 2 samples. With a 2-tailed test the probablility
statistic is 0.173 & with a 1-tailed it is 0.087. My interpretation is that
there is not a signigicant difference between the means using either the 1 or
2 tailed test. To be honest, I don't have a clue whether I should have used
a 1 or 2 tailed test so I did both. Can you also tell which I should be
using?
In another comparison of the means of 2 samples, the probability statistic
was 0.000346. I am interpreting this as a significant difference at the .03
CI. Is this correct?
gbh

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

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

 

[gbh]

Thank Jerry that really helped me. I should have been using the 2 tailed
test so I'll go with that probability. I assume I did correctly interpret
the probability of the second sample correctly that .000346 means a
significant difference in the groups at the .03 level. Thank again, my stats
classes were so long ago that I only know how to look up significance on a t
table knowing the degrees of freedom!
gbh

Most testing should be considered 2-tailed. The exception is if you can
honestly say yes to either of the following questions:

Before looking at the data, did you know which group MUST be larger if there
was a difference?

Before looking at the data, were you only interested in a difference if it
was in a particular direction (and it then turned out to be in that
direction)?

p=0.087 means that there is less than a 1 in 10 chance of observing such an
extreme difference (in the pre-specified direction) simply by chance if in
fact there is no difference. It is not significant at the 0.05 level, but
depending on your objectives, may be worth pursuing.

Jerry

Can you help me figure out if I'm interpreting the t-probability correctly?
I am comparing the means of 2 samples. With a 2-tailed test the probablility
statistic is 0.173 & with a 1-tailed it is 0.087. My interpretation is that
there is not a signigicant difference between the means using either the 1 or
2 tailed test. To be honest, I don't have a clue whether I should have used
a 1 or 2 tailed test so I did both. Can you also tell which I should be
using?
In another comparison of the means of 2 samples, the probability statistic
was 0.000346. I am interpreting this as a significant difference at the .03
CI. Is this correct?
gbh

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



:

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