Statistics Notes
What is the difference in the Chisquare and Variance graphs.
How does one change the "deviation" calculation to arrive at "variance"?
The Chisquare figures show the cumulative
deviation of the second-by-second Z-scores (squared),
compounded across the N eggs (N=36 to 38 at this time).
That is, for each second, the Z's for all the N eggs
are added and normalized by sqrt(N), then the resulting Z is
squared to yield a Chisquare with 1 df, and finally the
Chisquares-1 (Chisq=1 is the expectation) are cumulatively
summed, to represent the departure from expectation.
The Variance figures show something similar, but instead of
the compounded Z across eggs, the variance (squared standard
deviation) is computed across the N eggs for each second.
The sequence of Variance-50 (Var=50 is the expectation) is then
cumulatively summed as before.
The Chisquare figure displays extreme departures, in either
direction, of the trial scores of the egg from what is
expected by chance. The Variance figure displays the degree
of variability among the trial scores for the eggs. Chisquare
addresses movement of the central value of the distribution,
Variance represents changes in the range or width of the distribution.
What is the difference in the the analyses by Roger
Nelson and Dean Radin?
The most important difference is in the treatment of the data at the
finest scale. Neither way is superior, but there is a difference in what
is expected or hypothesized about the behavior of the eggs in the
presence of a possible influence. The two perspectives are
complementary, and though they are not fully independent, using both
contributes to our confidence that the apparent effects are not
accidents or mistakes.
For each second, Roger calculates what is called a
Stouffer Z across the eggs as described above.
This means that in order to produce a
large deviation, the eggs have to
have a positive correlation to be doing the same thing. This
composite Z is squared, so it does not matter whether the average value
is shifted to the high or low direction, but there must be some excess
deviation and there must be a tendency
toward inter-egg consistency in the direction of deviation.
The result is a single squared Z-score,
which is Chi-square distributed, for each second.
Dean calculates a Z-score for each egg separately, and squares these
individual Z-scores. He then sums the squared Z's across the eggs,
producing a a single Chi-square for each second. In this
case, the eggs are not expected to show a positive correlation, and a
high score requires only that there is a tendency for excess deviation
in either direction; no inter-egg consistency in the direction of
deviation is predicted. Again, the result is a single squared Z-score,
which is Chi-square distributed, for each second.