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    hi im doing a2 cw and it says on the mark scheme somethig about mentioning critical value. im doing chi squared and my observed value is 75.3 and on the chart it says 0.001. im using df2.
    however im not sure wot these facts mean as im crap at maths lol.
    cud some1 explain wot they are and tell me wot critical value is
    cheers
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    (Original post by emmz)
    hi im doing a2 cw and it says on the mark scheme somethig about mentioning critical value. im doing chi squared and my observed value is 75.3 and on the chart it says 0.001. im using df2.
    however im not sure wot these facts mean as im crap at maths lol.
    cud some1 explain wot they are and tell me wot critical value is
    cheers
    Okay, I'm not sure how far you've got so I'll start from scratch

    (A - B)^2
    chi-squared = E -----------
    B

    Where A= predicted value
    B= expected value
    E= sum of

    Once you have your chi-squared value from this equation (your critical value), you need to compare it to a chart. For this you need n (the number of categories in your data) - as you need to ascertain the degrees of freedom in order to work out what value you need to compare your data with.

    Degrees of freedom = n-1

    So if you had 4 categories, your degree of freedom would be 3.

    You also need to decide on a cut off point - with chi-squared you are assessing whether your data has deviated from the expected data by chance or due to a definite factor in your experiment. Usually a less than 5% probabilty of your data being down to interference is acceptable, thus you need to look for the column where p (probability) = 0.05.

    Now find the row where degrees of freedom matches what you have calculated for that, and find the value for that degree that is in the p=0.05 column. This is the value that you need to compare with your chi-squared critical value. If your critical value is LESS than the value in the table, you can conclude that statistically the deviations in your data from the expected values are due to random chance rather than a factor interfering.

    I know this is a horribly wordy and scientific information - it's tricky to explain, so if you need more info pm/reply me
 
 
 

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