The data for acetic acid is heavily negatively skewed and the pH is less skewed. Both are therefore non parametric.

Keep in mind please that there is no such thing as non-parametric data. There are non-

parametric test, though, which have less ssumptions than parametric tests (which for

example might assume normal, poisson, gamma etc. distribution of errors). Whether

you should use them, does not simply depend on skewness.

I know from box plots that there is a clear relationship of decreasing acetic acid content mean and median, smaller IQR and less outliers, as the rank of quality increases. So I am expecting a significant relationship. as you look at the quality ranks 1-10 (of which the 1600 wines are categorised) by increasing , acetic acid concentration decreases. As does pH.

If I understand you correctely, you want to correlate acid and pH with

quality. If we assume that quality ranking is an ordinal scaled variable,

then the Spearman rank correlation coefficient rho would be an option.

Alternatively, you could contemplate to take the logartithm of acid, and

maybe of pH, and perform Pearson-correlations with quality.

The interesting part could be to determine how acid and pH jointly predict

quality. This could be done e.g. by using multiple linear regression with log(acid),

pH, and the product log(acid) * pH as three predictors.

With kind regards

Karabiner