Charles Scripter writes:
BTW, I notice that your web page still seems to purport that your
analysis was correct, even though your friends over in sci.stat.edu
pointed out that it was not correct;
That’s an interesting interpretation of the discussion.
Perhaps you’d like to correct this “oversight”.
No problem, here’s something from one of my friends in sci.stat.edu:
Barry McDonald writes:
THE ARGUMENT ABOUT AUTOCORRELATION IN THE NSW HOMICIDE STATISTICS
One complaint by Scripter about your data was to do with the evident
autocorrelation in your data. It was this that was of interest to me.
You see sometimes autocorrelation is not real, but just apparent arising from
having omitted a variable from the analysis. The Minitab output shows this
is the
case for your data. First I should explain that a DurbinWatson statistic
shows significant autocorrelation if D< DL where DL is a number from tables,
clear nonsignificant autocorrelation if D>DU where DU is also from tables,
and inconclusive results if DL<= D <= DU.Suppose we just fit a single line to the data:
MTB > Regress 'homocide' 1 'year'; SUBC> Constant; SUBC> DW. Regression Analysis The regression equation is homocide = 47.3  0.0236 year Predictor Coef Stdev tratio p Constant 47.30 16.08 2.94 0.006 year 0.023646 0.008381 2.82 0.008 s = 0.5666 Rsq = 18.1% Rsq(adj) = 15.8% DurbinWatson statistic = 1.23Notice that if one just fits a straight line in terms of year then the
overall trend is downwards!! but the DurbinWatson statistic
indicates significant autocorrelation at the 5% level (D< DL=1.41) and
nearly at the 1% level (1.21).The autocorrelation is cleared up by fitting a more appropriate analysis
(see after graphs)MTB > GStd. MTB > Plot 'homocide' 'year'; SUBC> Symbol 'x'.Character Plot
3.20+  x x xx
homocide
 x x x
 x x
2.40+ x x
 x
 x x x x
 x x
 x x xx x
1.60+ x x x x
 x x x x x x
 x
 x
 x x x
0.80+

+++++year
1904.0 1911.0 1918.0 1925.0 1932.0MTB GPro.
MTB GStd.
MTB Plot 'FITS1' 'year';
SUBC Symbol 'x'.Character Plot
2.40+ x  xx
FITS1  xxx
 xx
 xx x
2.10+ xx
 xx x
 x xx
 x x
 x xx
1.80+ xx
 x xx
 xx
 xxx
 xx
1.50+ xx

+++++year
1904.0 1911.0 1918.0 1925.0 1932.0MTB GPro.
Allowing for a change in intercept level with the law change in 1920:
MTB > Regress 'homocide' 2 'year' 'lawchnge'; SUBC> Constant; SUBC> DW.Regression Analysis
The regression equation is homocide =  40.4 + 0.0223 year  1.18 lawchnge
Predictor Coef Stdev tratio p Constant 40.37 26.94 1.50 0.143 year 0.02233 0.01410 1.58 0.122 lawchnge 1.1769 0.3111 3.78 0.001
s = 0.4841 Rsq = 41.9% Rsq(adj) = 38.6%
DurbinWatson statistic = 1.63
There is clear evidence not to reject the hypothesis of zero
autocorrelation if D>DU=1.52. This is indeed the case so the addition of
this extra variable has simultaneously
given us a much more believable analysis (see two lines below: not going
down this time!!) and removed the apparent autocorrelation.
The choice of this cut point (1920) has had a very significant effect
(pvalue approx 0.001). Note however that the trend in year is not
significantly different to zero.MTB > GStd. MTB > Plot 'FITS2' 'year'; SUBC> Symbol 'x'.Character Plot
  xx x 2.40+ x xx x  xx x
FITS2  x xx x
 xx x
 x xx x
2.00+



 x xx
1.60+ xxx x
 x xx
 xxx x
 x xx

+++++year
1904.0 1911.0 1918.0 1925.0 1932.0If we just assume constant rates of homicides before 1921 and constant
after 1920, (i.e. zero slopes) then we retain a significant drop in
the level of homicides, and the autocorrelation is still not
significant. (though I would be cautious). The conclusion from this
test is exactly the same as you were trying to do by a twosample
ttest except that in this analysis we have the added feature of
checking whether the autocorrelation is significant.MTB >Regress 'homocide' 1 'lawchnge'; SUBC> Constant; SUBC> DW. Regression Analysis The regression equation is homocide = 2.28  0.753 lawchnge Predictor Coef Stdev tratio p Constant 2.2762 0.1078 21.11 0.000 lawchnge 0.7527 0.1612 4.67 0.000 s = 0.4941 Rsq = 37.7% Rsq(adj) = 36.0% Analysis of Variance SOURCE DF SS MS F p Regression 1 5.3221 5.3221 21.80 0.000 Error 36 8.7887 0.2441 Total 37 14.1108 Unusual Observations Obs. lawchnge homocide Fit Stdev.Fit Residual St.Resid 4 0.00 1.0000 2.2762 0.1078 1.2762 2.65R R denotes an obs. with a large st. resid. DurbinWatson statistic = 1.52(D= DU=1.52 so there is no proof of autocorrelation )
Of course this analysis does not clear up Scripter’s complaint that
(in his eyes) the law change year is irrelevant to homicides and so
an adjacent year could be used to give a similar significant
result. That is a causal matter that I cannot comment on as a
statistician. – except that since a highly significant effect is
apparent in the data, it really behooves him to come up with a
better explanation than yours, especially as to why he postulates
any other year as the change point.