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Subject: Regression to the Mean
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      <P align=3Dcenter><IMG height=3D60 alt=3D"Regression to the Mean"=20
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      <P><IMG height=3D457 hspace=3D10=20
      src=3D"http://trochim.human.cornell.edu/kb/images/regmean1.gif" =
width=3D350=20
      align=3Dright vspace=3D10> A regression threat, also known as a =
"regression=20
      artifact" or "regression to the mean" is a statistical phenomenon =
that=20
      occurs whenever you have a nonrandom sample from a population and =
two=20
      measures that are imperfectly correlated. The figure shows the =
regression=20
      to the mean phenomenon. The top part of the figure shows the =
pretest=20
      distribution for a population. Pretest scores are "normally" =
distributed,=20
      the frequency distribution looks like a "bell-shaped" curve. =
Assume that=20
      the sample for your study was selected exclusively from the low =
pretest=20
      scorers. You can see on the top part of the figure where their =
pretest=20
      mean is -- clearly, it is considerably below the population =
average. What=20
      would we predict the posttest to look like? First, let's assume =
that your=20
      program or treatment doesn't work at all (the "null" case). Our =
naive=20
      assumption would be that our sample would score just as badly on =
the=20
      posttest as they did on the pretest. But they don't! The bottom of =
the=20
      figure shows where the sample's posttest mean would have been =
without=20
      regression and where it actually is. In actuality, the sample's =
posttest=20
      mean wound up closer to the posttest population mean than their =
pretest=20
      mean was to the pretest population mean. In other words, the =
sample's mean=20
      appears to <I>regress toward the mean</I> of the population from =
pretest=20
      to posttest.</P>
      <H3><!--mstheme--><FONT color=3D#999933>Why Does It=20
      Happen?<!--mstheme--></FONT></H3>
      <P>Let's start with a simple explanation and work from there. To =
see why=20
      regression to the mean happens, consider a concrete case. In your =
study=20
      you select the lowest 10% of the population based on their pretest =
score.=20
      What are the chances that on the posttest that exact group will =
once again=20
      constitute the lowest ten percent? Not likely. Most of them will =
probably=20
      be in the lowest ten percent on the posttest, but if even just a =
few are=20
      not, then their group's mean will have to be closer to the =
population's=20
      posttest than it was to the pretest. The same thing is true on the =
other=20
      end. If you select as your sample the highest ten percent pretest =
scorers,=20
      they aren't likely to be the highest ten percent on the posttest =
(even=20
      though most of them may be in the top ten percent). If even just a =
few=20
      score below the top ten percent on the posttest their group's =
posttest=20
      mean will have to be closer to the population posttest mean than =
to their=20
      pretest mean.</P>
      <P>Here are a few things you need to know about the regression to =
the mean=20
      phenomenon: </P><!--mstheme--></FONT><!--msthemelist-->
      <TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" =
border=3D0><!--msthemelist-->
        <TBODY>
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
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            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica"><B>It is a =
<I>statistical</I>=20
            =
phenomenon.</B><!--mstheme--></FONT><!--msthemelist--></TD></TR><!--msthe=
melist--></TBODY></TABLE><!--mstheme--><FONT=20
      face=3D"arial, Arial, Helvetica">
      <BLOCKQUOTE>
        <P>Regression toward the mean occurs for two reasons. First, it =
results=20
        because you asymmetrically sampled from the population. If you =
randomly=20
        sample from the population, you would observe (subject to random =
error)=20
        that the population and your sample have the same pretest =
average.=20
        Because the sample is already at the population mean on the =
pretest, it=20
        is impossible for them to regress towards the mean of the =
population any=20
        more!</P></BLOCKQUOTE><!--mstheme--></FONT><!--msthemelist-->
      <TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" =
border=3D0><!--msthemelist-->
        <TBODY>
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
src=3D"http://trochim.human.cornell.edu/kb/_themes/kbtheme/posbul1a.gif" =

            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica"><B>It is a <I>group</I>=20
            =
phenomenon.</B><!--mstheme--></FONT><!--msthemelist--></TD></TR><!--msthe=
melist--></TBODY></TABLE><!--mstheme--><FONT=20
      face=3D"arial, Arial, Helvetica">
      <BLOCKQUOTE>
        <P>You cannot tell which way an individual's score will move =
based on=20
        the regression to the mean phenomenon. Even though the group's =
average=20
        will move toward the population's, some individuals in the group =
are=20
        likely to move in the other=20
      direction.</P></BLOCKQUOTE><!--mstheme--></FONT><!--msthemelist-->
      <TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" =
border=3D0><!--msthemelist-->
        <TBODY>
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
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            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica"><B>It happens between =
<I>any two=20
            =
variables.</I></B><!--mstheme--></FONT><!--msthemelist--></TD></TR><!--ms=
themelist--></TBODY></TABLE><!--mstheme--><FONT=20
      face=3D"arial, Arial, Helvetica">
      <BLOCKQUOTE>
        <P>Here's a common research mistake. You run a program and don't =
find=20
        any overall group effect. So, you decide to look at those who =
did best=20
        on the posttest (your "success" stories!?) and see how much they =
gained=20
        over the pretest. You are selecting a group that is extremely =
high on=20
        the posttest. They won't likely all be the best on the pretest =
as well=20
        (although many of them will be). So, their pretest mean has to =
be closer=20
        to the population mean than their posttest one. You describe =
this nice=20
        "gain" and are almost ready to write up your results when =
someone=20
        suggests you look at your "failure" cases, the people who score =
worst on=20
        your posttest. When you check on how they were doing on the =
pretest you=20
        find that they weren't the worst scorers there. If they had been =
the=20
        worst scorers both times, you would have simply said that your =
program=20
        didn't have any effect on them. But now it looks worse than that =
-- it=20
        looks like your program actually made them worse relative to the =

        population! What will you do? How will you ever get your grant =
renewed?=20
        Or your paper published? Or, heaven help you, how will you ever =
get=20
        tenured?</P>
        <P>What you have to realize, is that the pattern of results I =
just=20
        described will happen anytime you measure two measures! It will =
happen=20
        forwards in time (i.e., from pretest to posttest). It will =
happen=20
        backwards in time (i.e., from posttest to pretest)! It will =
happen=20
        across measures collected at the same time (e.g., height and =
weight)! It=20
        will happen even if you don't give your program or=20
      treatment.</P></BLOCKQUOTE><!--mstheme--></FONT><!--msthemelist-->
      <TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" =
border=3D0><!--msthemelist-->
        <TBODY>
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
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            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica"><B>It is a <I>relative=20
            =
</I>phenomenon.</B><!--mstheme--></FONT><!--msthemelist--></TD></TR><!--m=
sthemelist--></TBODY></TABLE><!--mstheme--><FONT=20
      face=3D"arial, Arial, Helvetica">
      <BLOCKQUOTE>
        <P>It has nothing to do with overall maturational trends. Notice =
in the=20
        figure above that I didn't bother labeling the x-axis in either =
the=20
        pretest or posttest distribution. It could be that everyone in =
the=20
        population gains 20 points (on average) between the pretest and =
the=20
        posttest. But regression to the mean would still be operating, =
even in=20
        that case. That is, the low scorers would, on average, be =
gaining more=20
        than the population gain of 20 points (and thus their mean would =
be=20
        closer to the =
population's).</P></BLOCKQUOTE><!--mstheme--></FONT><!--msthemelist-->
      <TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" =
border=3D0><!--msthemelist-->
        <TBODY>
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
src=3D"http://trochim.human.cornell.edu/kb/_themes/kbtheme/posbul1a.gif" =

            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica"><B>You can have regression =
up or=20
            =
down.</B><!--mstheme--></FONT><!--msthemelist--></TD></TR><!--msthemelist=
--></TBODY></TABLE><!--mstheme--><FONT=20
      face=3D"arial, Arial, Helvetica">
      <BLOCKQUOTE>
        <P>If your sample consists of below-population-mean scorers, the =

        regression to the mean will make it appear that they move =
<B><I>up=20
        </I></B>on the other measure. But if your sample consists of =
high=20
        scorers, their mean will appear to move <B><I>down</I></B> =
relative to=20
        the population. (Note that even if their mean increases, they =
could be=20
        losing ground to the population. So, if a high-pretest-scoring =
sample=20
        gains five points on the posttest while the overall sample gains =
15, we=20
        would suspect regression to the mean as an alternative =
explanation [to=20
        our program] for that relatively low =
change).</P></BLOCKQUOTE><!--mstheme--></FONT><!--msthemelist-->
      <TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" =
border=3D0><!--msthemelist-->
        <TBODY>
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
src=3D"http://trochim.human.cornell.edu/kb/_themes/kbtheme/posbul1a.gif" =

            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica"><B>The more extreme the =
sample group,=20
            the greater the regression to the =
mean.</B><!--mstheme--></FONT><!--msthemelist--></TD></TR><!--msthemelist=
--></TBODY></TABLE><!--mstheme--><FONT=20
      face=3D"arial, Arial, Helvetica">
      <BLOCKQUOTE>
        <P>If your sample differs from the population by only a little =
bit on=20
        the first measure, their won't be much regression to the mean =
because=20
        there isn't much room for them to regress -- they're already =
near the=20
        population mean. So, if you have a sample, even a nonrandom one, =
that is=20
        a pretty good subsample of the population, regression to the =
mean will=20
        be inconsequential (although it will be present). But if your =
sample is=20
        very extreme relative to the population (e.g., the lowest or =
highest=20
        x%), their mean is further from the population's and has more =
room to=20
        regress.</P></BLOCKQUOTE><!--mstheme--></FONT><!--msthemelist-->
      <TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" =
border=3D0><!--msthemelist-->
        <TBODY>
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
src=3D"http://trochim.human.cornell.edu/kb/_themes/kbtheme/posbul1a.gif" =

            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica"><B>The less correlated the =
two=20
            variables, the greater the regression to the=20
            =
mean.</B><!--mstheme--></FONT><!--msthemelist--></TD></TR><!--msthemelist=
--></TBODY></TABLE><!--mstheme--><FONT=20
      face=3D"arial, Arial, Helvetica">
      <BLOCKQUOTE>
        <P>The other major factor that affects the amount of regression =
to the=20
        mean is the correlation between the two variables. If the two =
variables=20
        are <I>perfectly</I> correlated -- the highest scorer on one is =
the=20
        highest on the other, next highest on one is next highest on the =
other,=20
        and so on -- there will no be regression to the mean. But this =
is=20
        unlikely to ever occur in practice. We know from measurement =
theory that=20
        there is no such thing as "perfect" measurement -- all =
measurement is=20
        assumed (under the <A=20
        href=3D"http://trochim.human.cornell.edu/kb/truescor.htm">true =
score=20
        model</A>) to have some random error in measurement. It is only =
when the=20
        measure has no random error -- is perfectly reliable -- that we =
can=20
        expect it will be able to correlate perfectly. Since that just =
doesn't=20
        happen in the real world, we have to assume that measures have =
some=20
        degree of unreliability, and that relationships between measures =
will=20
        not be perfect, and that there will appear to be regression to =
the mean=20
        between these two measures, given asymmetrically sampled=20
      subgroups.</P></BLOCKQUOTE>
      <H3><!--mstheme--><FONT color=3D#999933>The Formula for the =
Percent of=20
      Regression to the Mean<!--mstheme--></FONT></H3>
      <P>You can estimate exactly the percent of regression to the mean =
in any=20
      given situation. The formula is:</P>
      <P align=3Dcenter><B>P<SUB>rm</SUB> =3D 100(1 - r)</B></P>
      <P>where: </P>
      <BLOCKQUOTE>
        <P>P<SUB>rm</SUB> =3D the percent of regression to the mean<BR>r =
=3D the=20
        correlation between the two measures</P></BLOCKQUOTE>
      <P>Consider the following four =
cases:</P><!--mstheme--></FONT><!--msthemelist-->
      <TABLE cellSpacing=3D0 cellPadding=3D0 width=3D"100%" =
border=3D0><!--msthemelist-->
        <TBODY>
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
src=3D"http://trochim.human.cornell.edu/kb/_themes/kbtheme/posbul1a.gif" =

            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica">if r =3D 1, there is no =
(i.e., 0%)=20
            regression to the mean =
<!--mstheme--></FONT><!--msthemelist--></TD></TR><!--msthemelist-->
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
src=3D"http://trochim.human.cornell.edu/kb/_themes/kbtheme/posbul1a.gif" =

            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica">if r =3D .5, there is 50% =
regression to=20
            the mean =
<!--mstheme--></FONT><!--msthemelist--></TD></TR><!--msthemelist-->
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
src=3D"http://trochim.human.cornell.edu/kb/_themes/kbtheme/posbul1a.gif" =

            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica">if r =3D .2, there is 80% =
regression to=20
            the mean =
<!--mstheme--></FONT><!--msthemelist--></TD></TR><!--msthemelist-->
        <TR>
          <TD vAlign=3Dbaseline width=3D42><IMG height=3D20 hspace=3D11=20
            =
src=3D"http://trochim.human.cornell.edu/kb/_themes/kbtheme/posbul1a.gif" =

            width=3D20></TD>
          <TD vAlign=3Dtop width=3D"100%"><!--mstheme--><FONT=20
            face=3D"arial, Arial, Helvetica">if r =3D 0, there is 100% =
regression to=20
            the mean =
<!--mstheme--></FONT><!--msthemelist--></TD></TR><!--msthemelist--></TBOD=
Y></TABLE><!--mstheme--><FONT=20
      face=3D"arial, Arial, Helvetica">
      <P>In the first case, the two variables are perfectly correlated =
and there=20
      is no regression to the mean. With a correlation of .5, the =
sampled group=20
      moves <I><B>fifty percent</B></I> of the distance from the =
no-regression=20
      point to the mean of the population. If the correlation is a small =
.20,=20
      the sample will regress 80% of the distance. And, if there is no=20
      correlation between the measures, the sample will "regress" all =
the way=20
      back to the population mean! It's worth thinking about what this =
last case=20
      means. With zero correlation, knowing a score on one measure gives =
you=20
      absolutely no information about the likely score for that person =
on the=20
      other measure. In that case, your best guess for how any person =
would=20
      perform on the second measure will be the mean of that second =
measure.</P>
      <H3><!--mstheme--><FONT color=3D#999933>Estimating and Correcting =
Regression=20
      to the Mean<!--mstheme--></FONT></H3>
      <P><IMG height=3D509=20
      src=3D"http://trochim.human.cornell.edu/kb/images/regmean2.gif" =
width=3D350=20
      align=3Dleft> Given our percentage formula, for any given =
situation we can=20
      estimate the regression to the mean. All we need to know is the =
mean of=20
      the sample on the first measure the population mean on both =
measures, and=20
      the correlation between measures. Consider a simple example. Here, =
we'll=20
      assume that the pretest population mean is 50 and that we select a =

      low-pretest scoring sample that has a mean of 30. To begin with, =
let's=20
      assume that we do not give any program or treatment (i.e., the =
null case)=20
      and that the population is not changing over time on the =
characteristic=20
      being measured (i.e., steady-state). Given this, we would predict =
that the=20
      population mean would be 50 and that the sample would get a =
posttest score=20
      of 30 <I>if there was no regression to the mean</I>. Now, assume =
that the=20
      correlation is .50 between the pretest and posttest for the =
population.=20
      Given our formula, we would expect that the sampled group would =
regress=20
      50% of the distance from the no-regression point to the population =
mean,=20
      or 50% of the way from 30 to 50. In this case, we would observe a =
score of=20
      40 for the sampled group, which would constitute a 10-point =
pseudo-effect=20
      or regression artifact.</P>
      <P><IMG height=3D455=20
      src=3D"http://trochim.human.cornell.edu/kb/images/regmean3.gif" =
width=3D350=20
      align=3Dright> Now, let's relax some of the initial assumptions. =
For=20
      instance, let's assume that between the pretest and posttest the=20
      population gained 15 points on average (and that this gain was =
uniform=20
      across the entire distribution, that is, the variance of the =
population=20
      stays the same across the two measurement occasions). In this =
case, a=20
      sample that had a pretest mean of 30 would be expected to get a =
posttest=20
      mean of 45 (i.e., 30+15) if there is no regression to the mean =
(i.e.,=20
      r=3D1). But here, the correlation between pretest and posttest is =
.5 so we=20
      expect to see regression to the mean that covers 50% of the =
distance from=20
      the mean of 45 to the population posttest mean of 65. That is, we =
would=20
      observe a posttest average of 55 for our sample, again a =
pseudo-effect of=20
      10 points.</P>
      <P>Regression to the mean is one of the trickiest threats to =
validity. It=20
      is subtle in its effects, and even excellent researchers sometimes =
fail to=20
      catch a potential regression artifact. You might want to learn =
more about=20
      the regression to the mean phenomenon. One good way to do that =
would be to=20
      simulate the phenomenon. If you're not familiar with simulation, =
you can=20
      get a good introduction in the <A=20
      href=3D"http://trochim.human.cornell.edu/simul/simul.htm"><IMG =
height=3D44=20
      src=3D"http://trochim.human.cornell.edu/kb/images/dice2.gif" =
width=3D60=20
      border=3D0> The Simulation Book</A>. If you already understand the =
basic=20
      idea of simulation, you can do a <A=20
      href=3D"http://trochim.human.cornell.edu/simul/reg_m.htm"><IMG =
height=3D44=20
      src=3D"http://trochim.human.cornell.edu/kb/images/dice2.gif" =
width=3D60=20
      border=3D0> manual (dice rolling) simulation of regression =
artifacts</A> or=20
      a <A =
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      height=3D44 =
src=3D"http://trochim.human.cornell.edu/kb/images/dice2.gif"=20
      width=3D60 border=3D0> computerized simulation of regression =
artifacts</A>.=20
      =
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