A question on statistic t-test (Urgent)


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undergrd

Senior Member
Jun 16, 2007
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#1
Hi Ppl,

I need your help regarding the statistic t-test.

I have conducted a t-test on comparing 2 sample means and the following are the results:
Sample size 1 = 25, Mean 1 = 4.24, Standard Deviation 1 = 1.74.
Sample size 2 = 25, Mean 2 = 4.08, Standard Deviation 2 = 2.10.
Degree of Freedom = 48

Based on formula, I got a calculated t as 0.29.
From the t-distibution table based on degree of freedom = 48, level of significant = 0.05, two-tail, my table t-value is 2.011.

Comparing calculated t-value of 0.29 to table t-value of 2.011,
Does it mean that there is no significant difference in both sample? From the table am I able to find any critical upper and lower limit?

Thanks ppl....
 

Limsgp

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Dec 16, 2005
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#2
hi..

It's weird that such a question would pop up in a photography forum.. hehe.. but since it is kopitiam section, I'll make some wild guess and speculation..

Since the calculated value is 0.29, it implies that there is 29% probability that these 2 sample "have no significant difference".

Then, there'll be an hypothesis.. of the relationship between the 2 samples.

If the hypothesis is the 2 samples "have no significant difference", and the level is 0.05, then the hypothesis is true since 0.29 is > 0.05

If the hypothesis is the 2 samples "have significant difference", and the level is 0.05, then the hypothesis is false since 0.29 > 0.05
 

sheree

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Aug 25, 2007
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#3
wish i could help.. but i returned all my statistics knowledge to my math teacher when i finished my a's last year. HAHA i hate statistics. :(
 

night86mare

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Aug 25, 2006
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#4
hi..

It's weird that such a question would pop up in a photography forum.. hehe.. but since it is kopitiam section, I'll make some wild guess and speculation..

Since the calculated value is 0.29, it implies that there is 29% probability that these 2 sample "have no significant difference".

Then, there'll be an hypothesis.. of the relationship between the 2 samples.

If the hypothesis is the 2 samples "have no significant difference", and the level is 0.05, then the hypothesis is true since 0.29 is > 0.05

If the hypothesis is the 2 samples "have significant difference", and the level is 0.05, then the hypothesis is false since 0.29 > 0.05
er, t values have nothing to do with percentage, you need the read off from statistical tables to be able to tell what is the exact percentage, or estimations, anyways.
 

petetherock

Senior Member
Oct 9, 2006
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#5
Not sure how you got the value in bold

You can use this website:
http://www.graphpad.com/quickcalcs/ttest1.cfm

This is what I had:

Unpaired t test results

P value and statistical significance:
The two-tailed P value equals 0.7705
By conventional criteria, this difference is considered to be not statistically significant.

Confidence interval:
The mean of Group One minus Group Two equals 0.1600
95% confidence interval of this difference: From -0.9367 to 1.2567

Intermediate values used in calculations:
t = 0.2933
df = 48
standard error of difference = 0.545

P = 0.775

So it is greater than 0.05, hence the null hypothesis that the 2 samples are the same, stands.


t is NOT = p so 0.29 is NOT "29% probability that these 2 sample "have no significant difference".

Hi Ppl,

I need your help regarding the statistic t-test.

I have conducted a t-test on comparing 2 sample means and the following are the results:
Sample size 1 = 25, Mean 1 = 4.24, Standard Deviation 1 = 1.74.
Sample size 2 = 25, Mean 2 = 4.08, Standard Deviation 2 = 2.10.
Degree of Freedom = 48

Based on formula, I got a calculated t as 0.29.
From the t-distibution table based on degree of freedom = 48, level of significant = 0.05, two-tail, my table t-value is 2.011.

Comparing calculated t-value of 0.29 to table t-value of 2.011,
Does it mean that there is no significant difference in both sample? From the table am I able to find any critical upper and lower limit?

Thanks ppl....
 

night86mare

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Aug 25, 2006
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#6
I have conducted a t-test on comparing 2 sample means and the following are the results:
Sample size 1 = 25, Mean 1 = 4.24, Standard Deviation 1 = 1.74.
Sample size 2 = 25, Mean 2 = 4.08, Standard Deviation 2 = 2.10.
Degree of Freedom = 48

Based on formula, I got a calculated t as 0.29.
From the t-distibution table based on degree of freedom = 48, level of significant = 0.05, two-tail, my table t-value is 2.011.

Comparing calculated t-value of 0.29 to table t-value of 2.011,
Does it mean that there is no significant difference in both sample? From the table am I able to find any critical upper and lower limit?

Thanks ppl....
i cannot remember the formula for this particular scenario, i remember what it is roughly like, but assuming you applied the formula correctly,

and that t statstic is 0.29, for TWO TAILED TEST, with df = 48, significance level = 5%

then your t stat should lie between -2.011 and 2.011 for null hypothesis to NOT be rejected. null hypothesis is that there is no significant difference between the two sample means.
 

Last edited:

Pablo

Senior Member
Sep 1, 2004
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Blue/Green Planet
#7
Hi Ppl,

I need your help regarding the statistic t-test.

I have conducted a t-test on comparing 2 sample means and the following are the results:
Sample size 1 = 25, Mean 1 = 4.24, Standard Deviation 1 = 1.74.
Sample size 2 = 25, Mean 2 = 4.08, Standard Deviation 2 = 2.10.
Degree of Freedom = 48

Based on formula, I got a calculated t as 0.29.
From the t-distibution table based on degree of freedom = 48, level of significant = 0.05, two-tail, my table t-value is 2.011.

Comparing calculated t-value of 0.29 to table t-value of 2.011,
Does it mean that there is no significant difference in both sample? From the table am I able to find any critical upper and lower limit?

Thanks ppl....
Personally,

I would take a rough guess.

What are you talking about anyway :bsmilie:
 

Limsgp

New Member
Dec 16, 2005
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#8
Oops.. oh.. I had mistaken the T value for the P value.. hehe..


er, t values have nothing to do with percentage, you need the read off from statistical tables to be able to tell what is the exact percentage, or estimations, anyways.
 

undergrd

Senior Member
Jun 16, 2007
1,984
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#9
Hi Ppl, thanks for cracking your brains to help...

Sorry that this kind of qn was asked in a photography website..hehe.

Im able to understand bit by bit.Really appreciate all your help. Thanks thanks.
 

Dec 19, 2007
95
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Quahog, Rhode Island
#10
what is your t? 0.29?

what is your limit? 2.011?


thus, if you draw a bell curve with two lines on either side to mark out -2.011 and +2.011, 0.29 would fall in between the two lines. thus, H0 is rejected simply because it is not significant.

why is it not significant? because it does not fall outside the boundaries set out by your testing limits.

thus, you will report it as (t=0.29, p>0.05 or whatever alpha value you were told to use)





this proves that I have used SPSS so much for stats, I have completely forgotten all the theory behind the formulas/equations used by SPSS. :(
 

Last edited:

undergrd

Senior Member
Jun 16, 2007
1,984
7
38
North-East
#11
thus, if you draw a bell curve with two lines on either side to mark out -2.011 and +2.011, 0.29 would fall in between the two lines.
The 2.011 is obtained from the t-distribution table at 48 degree of freedom and 0.05 significant level. Just to ask, is it really true that the upper and lower critical value is -2.011 & 2.011? or do I have to manually calculate?
 

twinrongs

New Member
Apr 16, 2009
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#12
Calculate for you with PHStat software:

Data
Hypothesized Difference 0
Level of Significance 0.05
Population 1 Sample
Sample Size 25
Sample Mean 4.24
Sample Standard Deviation 1.74
Population 2 Sample
Sample Size 25
Sample Mean 4.08
Sample Standard Deviation 2.1

Intermediate Calculations
Population 1 Sample Degrees of Freedom 24
Population 2 Sample Degrees of Freedom 24
Total Degrees of Freedom 48
Pooled Variance 3.7188
Difference in Sample Means 0.16
t Test Statistic 0.293341547

Two-Tail Test
Lower Critical Value -2.010634722
Upper Critical Value 2.010634722
p-Value 0.77052566
Do not reject the null hypothesis
i.e. There is no significant difference between the 2 means. Null hypothesis: X1 = X2

Confidence Interval Estimate
for the Difference Between Two Means

Data
Confidence Level 95%

Intermediate Calculations
Degrees of Freedom 48
t Value 2.010634722
Interval Half Width 1.09667914

Confidence Interval
Interval Lower Limit -0.93667914
Interval Upper Limit 1.25667914
 

Dec 19, 2007
95
0
0
Quahog, Rhode Island
#13
The 2.011 is obtained from the t-distribution table at 48 degree of freedom and 0.05 significant level. Just to ask, is it really true that the upper and lower critical value is -2.011 & 2.011? or do I have to manually calculate?
well based on the information given by you in your initial post, I assume that the upper and lower limits of your two tailed test is +/- 2.011

how these numbers are arrived at, I have no idea. if I remember correctly, the intervals or something has got something to do with it... 1.96 or something. I'm not sure.
 

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