Having Trouble Understanding Chi Squared Stuff

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Discussion Overview

The discussion revolves around understanding the chi-squared statistical method, particularly in the context of fitting data from an e/m lab experiment. Participants are exploring how to calculate standard deviations for their data and the concept of degrees of freedom in their analysis.

Discussion Character

  • Exploratory
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant expresses confusion about which standard deviation to use for their chi-squared calculations, noting they have three different measured quantities.
  • Another participant suggests that if all standard deviations are the same, they can be factored out of the sum, but they are unclear on the specifics.
  • A participant later proposes using the standard deviation from the averages of five sets of data for their five data points.
  • There is uncertainty regarding the calculation of degrees of freedom, with one participant suggesting it should be 5 - 3 = 2 based on their data points and measured quantities.
  • A participant humorously claims to have solved their confusion by fabricating data to achieve aesthetically pleasing results.
  • Links to external resources on chi-squared tests are provided for further reference.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the correct approach to calculating standard deviations or degrees of freedom, indicating that multiple competing views and uncertainties remain in the discussion.

Contextual Notes

Participants express confusion over the notation and specific definitions related to standard deviation and degrees of freedom, which may affect their calculations and understanding of the chi-squared method.

Poop-Loops
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So I have my fit for my data (in this case it's the e/m lab, where the slope of my trend line = e/m).

And I calculated the required y - (Ax + B) values, but I have no idea what to use for standard deviation to divide each value by. To be honest, I'm just lost in the notation. I remember in the lecture hearing that if all the STD's are the same, you can pull that out of the sum, but I have no idea which STD's he was talking about, since I have 3 different measured quantities.

Also, degrees of freedom. Since I used 5 data points and had to measure 3 different quantities for each point, I would use D = 5 - 3 = 2?
 
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I figured it out! I just make up the data to make it look pretty! That's how you get all the grant money.
 
No, seriously, I could still use some help.

I think I figured out which STD to use. Since my y-values are averages from 5 sets of data to get 5 data points, I just use the STD for the 5 different sets of data as my 5 different STD's, right?

I'm still not sure I understand the degree's of freedom, though.
 

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