# Please help!! How do I CALCULATE the expected value? Chi square test!!

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Thread starter 4 years ago
#1
I know this is biology, but no one answered me in the biology forum, and cos the Chi square test is also part of Maths, I thought I might hopefully get some responses here.

An experiment was carried out to investigate the inheritance of grain shapes and colour in maize. The number of each type of grain on a cob were counted (observed number).
The results are shown in the table below:

it was predicted that the phenotype would be in the ratio 9 : 3 : 3: 1 and a chi-squared test was used to investigate whether there was a significant difference between the observed results and the expected results.

Complete the table by calculating the expected number (E) of grains of each phenotype.

--

So what I did was:
the ratio is 9 : 3 : 3 : 1

For yellow & smooth: 9/16 x 53 = 29.8125
Yellow & wrinkled: 3/16 x 20 = 3.75
and so on ...

but my answers are all wrong?? Where did I go wrong and what's the correct way to calculate the expected value?
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4 years ago
#2
(Original post by Adorable98)
^
You've got the right sort of idea.

But you want to multiply the expected ratio (i.e. 9/16) by the TOTAL count (53 + 20 + 17 + 10 = 100)

So they should be:

yellow/smooth: 9/16 * 100 = 56.25
yellow/wrinkled: 3/16 * 100 = 18.75
white/smooth: 3/16 * 100 = 18.75
white/wrinkled: 1/16 * 100 = 6.25

You can see that these are much closer to the observed values.

I'm sure you can fill out the last column yourself from that
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Thread starter 4 years ago
#3
(Original post by Alexion)
X
Aha, now I get it, Thanks!!
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4 years ago
#4
(Original post by Adorable98)
Aha, now I get it, Thanks!!
Can you see that when you do it via Alexion's way, the sum of your expected values sum to 100, as expected?
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Thread starter 4 years ago
#5
(Original post by Zacken)
Can you see that when you do it via Alexion's way, the sum of your expected values sum to 100, as expected?
Yes, but should the sum always add up to a 100 ?
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4 years ago
#6
(Original post by Adorable98)
Yes, but should the sum always add up to a 100 ?
No, it should always add up to whatever your observed things add up to.
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Thread starter 4 years ago
#7
(Original post by Zacken)
No, it should always add up to whatever your observed things add up to.
Oh, I see!! Thanks
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4 years ago
#8
(Original post by Adorable98)
Oh, I see!! Thanks
No worries.
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