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Thread: SPSS: transforming variables?

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    SPSS: transforming variables?




    I am doing a single linear regression and multiple regression, but I have a question about my independent variable.

    My independent variables (X) are between 0 and 100.
    Some of my hypothesizes is that "A high X will lead to a lower Y"

    High X = value above fifty to hundred
    and Low x = value from 0 till 49

    I assume I have to change my X in SPSS so it corresponds to high and low?
    How do I do this? Or am I completely wrong?

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    Re: SPSS: transforming variables?

    Quote Originally Posted by Jazz3 View Post
    I am doing a single linear regression and multiple regression, but I have a question about my independent variable.

    My independent variables (X) are between 0 and 100.
    Some of my hypothesizes is that "A high X will lead to a lower Y"

    High X = value above fifty to hundred
    and Low x = value from 0 till 49

    I assume I have to change my X in SPSS so it corresponds to high and low?
    How do I do this? Or am I completely wrong?
    You don't necessarily need to change it. I probably wouldn't. Here's why: 1) how did you determine that 50 or higher was "high"? If this is an arbitrary cutoff assigned without evidence to support the cutoff, this isn't really a good idea (you'll unnecessarily lose information and changing the cutoff can give different results). 2) If you leave X as is, 0-100, you can just test that its coefficient is less than 0, implying that as X increases, Y decreases, on average (which would answer your question).

    Another thing to consider is the nature of the relationship.

    1)Do you think that above some threshold (like 50), Y declines with a "jump"-- i.e. a discrete change (think of a stair case with only 1 step for this example)? This would imply that there isn't much change for values within the same group, but between the groups there is a noticeable difference.
    -OR-
    2) Do you think that the change in Y is more gradual when you change X? In other words, even though 50, 55, and 60 might all fall into what you would consider a "high" group, there is a noticeable and gradual decline in Y as you increase from 50,51,52...55...60... This would also more fit with the idea that going from 49 to 50 is not some discrete jump.

    If number 1 is the case, then sure, make categories out of the X variable, but recognize the loss of information. Be sure that your cutoff can be justified with subject matter knowledge/theory rather than "well, I thought this would be a good cutoff."

    If number 2 is the case, just use X as it is, without transformation. Do a one-tailed test for beta1 < 0 to test if Y decreases as X increases.
    Last edited by ondansetron; 02-16-2017 at 04:09 PM. Reason: Typos...

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    Re: SPSS: transforming variables?


    Thank you for your reply! This made it very clear

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