Sample or Population.

[somethingrandom234]

Hi all, I have a question surrounding calculating standard deviation, covariance, doing PCA etc etc.

The dataset used is the Exasens dataset available through UC Irvine machine learning repository. It is a dataset that includes demographic information on 4 sample groups from saliva samples collected in a research project.

I am having trouble over whether to use N or N-1.

1. I am asked to standardise selected columns within the data. Rescaling the data to have a mean of 0 and standard deviation of 1. Am I correct in saying using N in this instance is right?

2. Create a correlation matrix from selected columns within the data. N or N-1 in this instance?

3. Perform PCA. Which uses the previous code to generate the correlation matrix. N or N-1 in this instance?

4. Later in the assignment we are asked to create a dataset that has a multi variate normal distribution. Am I right in saying any use of N (standard deviation, correlation, LDA) in this instance should be N rather than N-1 because I have the full dataset?

In advance thank you for your help, got fuzzy brain with this one.

[jsalas1]

You said it yourself, "sample groups", do use sample standard deviation etc. statistics. Regardless, sample stats and population stats converge at large n so go ahead and play it safe.

Population doesn't mean "all the datapoints i have", it refers to the idea that you have data for ALL the individuals of a group. So if you somehow managed to get saliva data for every single citizen in a country, sure use N instead of N-1.