Hey there! Sign in to join this conversationNew here? Join for free
    • Thread Starter
    Offline

    2
    ReputationRep:
    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) My spec 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? What else does it mean? ("Coding" is confusing me.)

    Thanks a lot.
    • Thread Starter
    Offline

    2
    ReputationRep:
    Any help?
    Offline

    11
    ReputationRep:
    (Original post by Big-Daddy)
    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) My spec 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? What else does it mean? ("Coding" is confusing me.)

    Thanks a lot.
    a) Yes and Yes.
    b) Translation does not affect Spearman's Rank, provided everything is translated the same amount.
    See mathematical properties for the PPMCC http://en.wikipedia.org/wiki/Pearson_product-moment_correlation_coefficient
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by joostan)
    a) Yes and Yes.
    b) Translation does not affect Spearman's Rank, provided everything is translated the same amount.
    See mathematical properties for the PPMCC http://en.wikipedia.org/wiki/Pearson_product-moment_correlation_coefficient
    Thanks

    What sort of graph would have a Spearman's rank which is not equal to 1 but a PPMCC which is roughly equal to 1? (Question in my textbook to which I don't know the answer)
    Offline

    11
    ReputationRep:
    (Original post by Big-Daddy)
    Thanks

    What sort of graph would have a Spearman's rank which is not equal to 1 but a PPMCC which is roughly equal to 1? (Question in my textbook to which I don't know the answer)
    All y increasing as x increases, (i.e. correlated) except one which is different perhaps?
    • Thread Starter
    Offline

    2
    ReputationRep:
    (Original post by joostan)
    All y increasing as x increases, (i.e. correlated) except one which is different perhaps?
    That's what I was thinking, I guess it comes down to whether that will impact the PMCC much or not.

    I just tried a case where 1 variable out of 7 was changed and the PMCC dropped from 1 to 0.83 so the impact is pretty significant. Obviously once the order is changed from exact Spearman's Rank is no longer exactly 1. Do you think this fits the roughly equal to 1 criterion though?
    Offline

    11
    ReputationRep:
    (Original post by Big-Daddy)
    That's what I was thinking, I guess it comes down to whether that will impact the PMCC much or not.

    I just tried a case where 1 variable out of 7 was changed and the PMCC dropped from 1 to 0.83 so the impact is pretty significant. Obviously once the order is changed from exact Spearman's Rank is no longer exactly 1. Do you think this fits the roughly equal to 1 criterion though?
    You can't get it any nearer than only one variable changed.
    Obviously if there were more variables, the effect would be less marked.
 
 
 
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • Poll
    Have you ever participated in a Secret Santa?
    Useful resources

    Make your revision easier

    Maths

    Maths Forum posting guidelines

    Not sure where to post? Read the updated guidelines here

    Equations

    How to use LaTex

    Writing equations the easy way

    Student revising

    Study habits of A* students

    Top tips from students who have already aced their exams

    Study Planner

    Create your own Study Planner

    Never miss a deadline again

    Polling station sign

    Thinking about a maths degree?

    Chat with other maths applicants

    Can you help? Study help unanswered threads

    Groups associated with this forum:

    View associated groups
  • See more of what you like on The Student Room

    You can personalise what you see on TSR. Tell us a little about yourself to get started.

  • The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

    Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

    Quick reply
    Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.