# Stats Problems with PMCC

1. Apr 5, 2013

Edit: I had no idea what I was doing when I posted this here, sorry! Could someone move it to the Maths help forum?

I have a few basic stats questions which I'd appreciate if someone could help me answer. These are conceptual really.

a) Often I'm asked to come up with a graph which has a Spearman's Rank coefficient of 1 but a PMCC which is not 1. For such a case, would it be acceptable to use an exponential-type curve (just sketching, talking about the shape) - as it wouldn't fit a straight line (so PMCC is not 1) but if you move to a higher x-value you will still always get a higher y-value? If Spearman's rank has to be -1 but the PMCC not quite -1, can I use a downward sloping graph of increasing negative gradient?

b) What does a graph with Spearman's Rank≠1 but PMCC≈1 look like? (If such a graph exists)

b) My syllabus says "understand that the value of a correlation coefficient is unaffected by linear transformations (coding) of the variables". Does this mean that if you translate the scatter points, it will not affect either the PMCC or Spearman's rank? ("Coding" is confusing me.) And any other transformation besides translation will affect both PMCC and Spearman's Rank (e.g. stretch in either axis) ...

Thanks :)

Last edited: Apr 5, 2013