# Thread: Calculating a value from a trendline.

1. ## Calculating a value from a trendline.

After using the following numbers to create a calibration curve
X
133406
418832
844875

y
0.0408
0.2221
0.4341

and adding a trendline the equation of the trendline is is
y = 0.000000548500693x - 0.023105816308584

How is it possible to use this to calculate the Y values for these X values?
39231.3

When I tried for the first number
y = 0.000000548500693(39231.3) - 0.02311
y = 0.0215183952372909 - 0.02311
y = -0.00159160 which can't be possible
when i'm guessing the answer should be around 0.0130

Thank you in advance

2. Your trendline is correct and your calculation is correct too.

Why do you expect it to be 0.0130?

3. From using the calibration numbers, it's around 1/3th of the first numbers
133406 which gave a y value of 0.0408
so 0.0400 / 3

and the values are absorbency (x) against concentration (y) so for an absorbency value of 39231.3 to give a concentration of -0.00159160 isn't possible

Would it mean that the 39231.3 value is just incorrect?

4. I think that you'll have instead to force your trendline to pass through (0, 0)

I'm not sure about absorbency and concentration, but would concentration be 0 if the absorbency is 0? If yes, then you'll have to force it to (0, 0)

And from there, you will use this new trendline to get the other values.

5. I've got the trendline going though x which is showing a concentration of 0 giving an absorbency value of around 45000

which gives the same trendline equation so the value of

y = -0.00159160 for 39231.3 absorbency

Which looks like the values for absorbency (39231.3) was just measured incorrectly?

6. Well, if the trendline passes through (0, 0), then the equation will be in the form:

$\displaystyle y = mx$

only.