# Search results

1. ### I Right way to do a linear fit

Please find the data I am using below. The errors are combined statistical and systematic. They come from different experiments (hence the different range of errors). Just to give a bit more details, the function I actually need to fit is this ##z=y(a+b(x+0.5)/4.186)## (just a redefinition of a...
2. ### I Right way to do a linear fit

Oh I see! So if the fit is good b and d should be consistent with zero, right? Thanks a lot! Could you please explain to me a bit more why doing it in 2 steps gives me a different error (it is actually ~3 times smaller)?
3. ### I Right way to do a linear fit

Thank you for your reply! To be honest I wasn't even sure if they can count as different, I assumed they are the same method, but it one case it do it in 2 steps while in the other in one step only. The errors on y are a lot smaller than the errors on z. From what I've seen ignoring them...
4. ### I Right way to do a linear fit

The data is from a molecular spectroscopy experiment. For people working in the field, this is similar to a King plot fit, but for molecular terms (when the field shift is important). z corresponds to a frequency shift between different molecules, y is the change in radius of one of the atoms of...
5. ### I Right way to do a linear fit

Hello! I have some data of the form (x,y,z) which I know it is described by a function of the form: ##z=y(a+bx)##, where a and b are parameters to be fitted for. z and y have error associated to them while x doesn't (x is actually an integer going from 0 to 3 for each value of y). I tried to do...
6. ### I Bad point in a linear fit

The x axis represents some fixed property of the measured element. It is a bit complicated to describe, but you can think of it as something associated to the element that you can't modify (such as the atomic number). The y axis can be thought of as a transition frequency of that measured...
7. ### I Bad point in a linear fit

oh sorry, I got confused by "report the values". I need to report just one value: the slope. I assume you meant I have to take into account all the values for the fit, right?
8. ### I Bad point in a linear fit

For the purpose of my question you can ignore the error on the x axis (you can assume it is zero). The counts are just related to the y-error. Also I am still confused by what you mean by "values" and "all data". What I need is just a number: the value of the slope (and its error). Why would I...
9. ### I Bad point in a linear fit

What do you mean by report the values. Values for what? Also I am not sure about the size of the error bars. I assume in certain cases the counts were just higher so the statistical error was lower and maybe they had a better control on their systematics. But my main question is about the 3rd...
10. ### I Bad point in a linear fit

It was just one measurement for each data points. It was a counting experiment, from which they got the statistical error ##\sigma_{stat}##, but also some systematics error (having to do with the experimental setup), ##\sigma_{syst}##. The error on the points is...
11. ### I Bad point in a linear fit

Before plotting it, no, I didn't suspected that point is bad. That experiment was done quite a long time ago, so it's difficult to point towards an issue (but most probably it is some systematic error they didn't account for). The error bars come from adding in quadrature the systematic and...
12. ### I Bad point in a linear fit

Hello I have these points coming from different experiments: ##x = [-0.3, -0.2, -0.09, 0.01, 0.2]## ##y = [-8.15, -5.20, -3.32, 0., 5.65]## ##y_{err} = [0.1, 0.27, 0.35, 0.09, 0.44]## and I need to fit a straight line to them (based on theoretical arguments). I attached below the obtained...
13. ### I How to calculate the right chisquare

What do you mean by expected number? Isn't that the number of events?
14. ### I How to calculate the right chisquare

Hello! I have some simulated background only data and some background plus signal simulated data. After some cuts I end up with a histogram for each of the 2 sets and I want to calculate the chisquare (and hence the p-value for a signal actually being present in the background plus signal...
15. ### I Bin center location

So basically, whether the error on the freq is significant or not, rebinning will not help?
16. ### I Bin center location

As I said I can't post my data. Also that peak is visible without rebinning, I am worried that the uncertainty in the freq measurement would add to the error of the actual peak center and I am not sure how to account for that yet.
17. ### I Bin center location

I made some clarifications in a post after, those are rates, not counts, I am sorry. So we used a laser to ionize some molecules and counted the number of ionized molecules for each laser wavelength. The laser (Ti:Sapphire) wavelength was adjusted with a predefined schedule and when the...
18. ### I Bin center location

I am sorry, yes I meant rate (to get counts multiply the first 2 columns, my bad!). Also I have few tens of thousands of points between 13252.3231 and 13293.9497 ##cm^{-1}##. Here I provided some randomly selected points just to give an example the kind of data that I have, but there is a peak...
19. ### I Bin center location

Here is some of the raw data (also thanks a lot for helping me with this). The columns are measurement time is seconds, number of counts and wavenumber in ##cm^{-1}##. 1.10898922 385.035301 13252.7038 1.10137726 322.323705 13252.7067...
20. ### I Bin center location

What confuses me about adding the rates directly the way you suggest, is that the final rate (i.e. the sum) will reflect something of the form counts per second per bin width (I think). So doubling the bin width and measuring everything that come into that bin in a given amount of time would...
21. ### I Bin center location

So I agree that if you measure them separately you would get r1 + r2, but this is not what binning is. When you do a rebinning in a rate vs energy plot, you basically show the values of the rates at different points than before (the middle of the points from before, if using double the size of...
22. ### I Bin center location

Sorry I am still confused. First of all, I might not have explained my approach well. Given your example I would get (2000+4000)/(100+10) = 54.5 counts/second. I am not sure how to get 38.2, but that is not what I tried to explain and given that I have 20 counts on the left and 400 on the right...
23. ### I Bin center location

But you also have 20 counts/sec in one of the bins, so I don't see the problem. But let's take a super basic example. Assume that I have a bin between 5 and 15 (in some energy units), centered at 10, with 100 counts in 1 second. The next bin is between 15 and 25, centered at 20, also with 100...
24. ### I Confidence intervals

Thank you for your reply! Actually I finally understand why using the error matrix is bad. However I am still a bit confused about how Bevington calculates the confidence intervals. He uses a chi squared distribution and for example he uses the fact that for one parameter an increase on 1 for...
25. ### I Bin center location

Yeah, that's what I am saying. Adding the rates directly doesn't make sense to me.
26. ### I Confidence intervals

Hello! Can someone help me understand how are confidence intervals for some parameters of a fit different from the errors on the parameters obtained, for example, from the error matrix. I read Bevington and the whole book he mentions that we can use the error from the error matrix to define the...
27. ### I Bin center location

I am not sure I understand why would I add the rates together. If my both rates are 100 counts in 10 seconds (i.e. each of them are 10 counts per second), if I combine them I get 200 counts in 20 seconds which gives a count also of 10 counts per second. Based on what you said, adding the counts...
28. ### I What error to use on the mean

Hello! I have an experiment and for some reasons I was able to do only 4 measurements and they all ended up having the same value, say for the purpose of this post ##100 \pm 1## where the error of 1 is estimated based on the measuring device resolution. The mean is obviously 100. Usually the...
29. ### I Bin center location

Thank you for the clarification! You're right, merging bins so different is not a great idea. My questions was mostly for bins with similar counts, in case I want to combine them (for example 98, 100, 103 counts per second). I gave that example just to easily do the computations. So I agree with...
30. ### I Bin center location

Thank you for your reply! I am a bit confused about the way you calculate the rate when rebinning. Why do you use the error in that calculation. To use a toy example numbers: if I know that I have 100 counts in 10 seconds between 0 and 50 (in some arbitrary energy units) and 2000 counts in 100...