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I have an assignment where I need to write a function between two variables:

Write a function that takes a directory of data files and a threshold for complete cases and calculates the correlation between sulfate and nitrate for monitor locations where the number of completely observed cases (on all variables) is greater than the threshold. The function should return a vector of correlations for the monitors that meet the threshold requirement.

The part I am confused about is "threshold". I don't understand how "threshold" relates to finding the correlation between the two variables. Can someone help me understand or point me to where I can find more information?

Thank you.

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This seems to refer to a conditional filter whereby the correlation is calculated only for datasets which have fewer than a certain number (threshold) of missing values (ie if there are fewer than x complete observations, the monitor is omitted). Makes sense for very large datasets to make sure you don't get correlations if there are unreasonably few observations.

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  • $\begingroup$ Thanks for your answer. As a follow up question, would the threshold equal to 'N' (sample size) in the equation for covariance? If so, then how would I choose which values to put in my sample size? Thanks! $\endgroup$ Commented Nov 13, 2014 at 5:27
  • $\begingroup$ I think this problem calls for a threshold as an argument in your function so that you could specify it when you call the function. Unless it is the min of complete cases in all datasets, not really sure from the way it is stated. $\endgroup$
    – katya
    Commented Nov 13, 2014 at 17:48

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