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### Topic: set the intercept to zero  (Read 4627 times)

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#### grs35

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##### set the intercept to zero
« on: March 24, 2012, 11:26:08 AM »
i am using Excel to calculate standard curves from absorbance data. i have 4 concentrations matching with 4 different absorbance vaules. but i have to set the graph to zero first in order to write the equation.

because right now the line looks crooked. itis not linear. and therefore i have 3 different slopes. i have to have 1 single slope so that i can apply this equation to other calculations.

anyone knows how to set the graph to zero and write down this equation ?

here are the data :

x values
2.10^-6
4.10^-6
6.10^-6
1.10^-5

y values
0.169
0.342
0.489
0.795

#### Borek

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##### Re: set the intercept to zero
« Reply #1 on: March 24, 2012, 11:52:08 AM »
Shouldn't you do a linear regression on your data?
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#### grs35

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##### Re: set the intercept to zero
« Reply #2 on: March 24, 2012, 12:31:32 PM »
Shouldn't you do a linear regression on your data?

honestly i am confused because of these results.

#### Borek

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##### Re: set the intercept to zero
« Reply #3 on: March 24, 2012, 01:56:48 PM »
Looks quite linear to me. Have you done the blank test to see what is the absorbance for zero concentration? I would not force the curve to go through zero just for the sake of doing it.
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#### grs35

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##### Re: set the intercept to zero
« Reply #4 on: March 24, 2012, 04:29:41 PM »
Looks quite linear to me. Have you done the blank test to see what is the absorbance for zero concentration? I would not force the curve to go through zero just for the sake of doing it.

i put the values on the graph and its not linear. there are three slopes 86500, 73500, 76500

yes, i did it for zero concentration too. but they want us to perform linear regression and write down the equation. but it is impossible to do that with a nonlinear graph.

honestly i dont want to force it to go through zero too. but what should i do then ?

#### Arkcon

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##### Re: set the intercept to zero
« Reply #5 on: March 24, 2012, 05:35:07 PM »
Maybe if you showed us your chart, we could see where you're having a problem.  Yes, there are many possible slopes, but a linear regression will give you the best possible one to take in to account all that you know ... for most data sets likely found in an undergraduate laboratory.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

#### grs35

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##### Re: set the intercept to zero
« Reply #6 on: March 24, 2012, 05:49:21 PM »
Maybe if you showed us your chart, we could see where you're having a problem.  Yes, there are many possible slopes, but a linear regression will give you the best possible one to take in to account all that you know ... for most data sets likely found in an undergraduate laboratory.

here is what i got.

bluedye1.numbers

#### Borek

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##### Re: set the intercept to zero
« Reply #7 on: March 24, 2012, 05:57:56 PM »
Please post it as an attached image.
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#### grs35

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##### Re: set the intercept to zero
« Reply #8 on: March 24, 2012, 06:19:40 PM »

#### grs35

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##### Re: set the intercept to zero
« Reply #9 on: March 24, 2012, 06:23:37 PM »
i think this works.

#### Borek

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##### Re: set the intercept to zero
« Reply #10 on: March 24, 2012, 06:26:35 PM »
Looks to me like you plotted absorbance against point number, not against concentration. Make a scatter plot.
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#### grs35

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##### Re: set the intercept to zero
« Reply #11 on: March 24, 2012, 06:41:17 PM »
Looks to me like you plotted absorbance against point number, not against concentration. Make a scatter plot.

#### blaisem

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##### Re: set the intercept to zero
« Reply #12 on: March 24, 2012, 07:27:32 PM »
Look at your y-axis.  This is your absorbance. Do you see how each one of your data points matches an absorbance on your y-axis?  Now look at your x-axis.  This is your concentration.  Does each data point match one of your concentrations?  They don't.  You'll notice the x-axis value for each of your data points is 1, 2, 3, 4.  They should be 0.000002, 0.000004, 0.000006, 0.00001.  If they don't have that on the x-axis, then you set your graph up wrong.

It looks like you didn't specify values for your x-axis, so Excel just filled in generic values 1, 2, 3, 4.  You need to specify values for the x-axis.  To do this, right click on one of your data points, then click "select data."  There is an option there to edit a series, and you can then pick your x-axis and y-axis values as you please.  Try that out.

It does look like you got your trendline down, right, though.  So that's good.  You just need to fix your x-axis.
« Last Edit: March 24, 2012, 07:47:28 PM by blaisem »

#### grs35

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##### Re: set the intercept to zero
« Reply #13 on: March 26, 2012, 08:05:57 PM »
Look at your y-axis.  This is your absorbance. Do you see how each one of your data points matches an absorbance on your y-axis?  Now look at your x-axis.  This is your concentration.  Does each data point match one of your concentrations?  They don't.  You'll notice the x-axis value for each of your data points is 1, 2, 3, 4.  They should be 0.000002, 0.000004, 0.000006, 0.00001.  If they don't have that on the x-axis, then you set your graph up wrong.

It looks like you didn't specify values for your x-axis, so Excel just filled in generic values 1, 2, 3, 4.  You need to specify values for the x-axis.  To do this, right click on one of your data points, then click "select data."  There is an option there to edit a series, and you can then pick your x-axis and y-axis values as you please.  Try that out.

It does look like you got your trendline down, right, though.  So that's good.  You just need to fix your x-axis.

thank you everyone. It was so simple but sometimes the mind just stops here is what i got at last. and its correct.