Dear Stata users:
I define two types of firms domestic ones (type 1) and exporting ones (type 0).
I want to test whether type 0 firms have higher average productivity
of labour (APL) than type 1, which graphical evidence seems to show by
plotting firm-level productivity distributions.
I run two tests:
ranksum l_APL, by(type_firm) porder
ksmirnov l_APL, by(type_firm)
Stata outcomes:
Two-sample Wilcoxon rank-sum (Mann-Whitney) test
type_firm | obs rank sum expected
-------------+---------------------------------
0 | 761 376455 364138.5
1 | 195 80991 93307.5
-------------+---------------------------------
combined | 956 457446 457446
unadjusted variance 11834501
adjustment for ties -.16253916
----------
adjusted variance 11834501
Ho: l_APL(type_f~m==0) = l_APL(type_f~m==1)
z = 3.580
Prob > |z| = 0.0003
P{l_APL(type_f~m==0) > l_APL(type_f~m==1)} = 0.583
Two-sample Kolmogorov-Smirnov test for equality of distribution functions:
Smaller group D P-value Corrected
----------------------------------------------
0: 0.0185 0.899
1: -0.1755 0.000
Combined K-S: 0.1755 0.000 0.000
I am confused:
Wilcoxon rank-sum (Mann-Whitney) test implies that type 0 firms have
higher productivity :
P{l_APL(type_f~m==0) > l_APL(type_f~m==1)} = 0.583
Whereas KS test shows that it is the opposite (.899 as a pvalue when
testing that type 0 is smaller).
I guess that there is something I am misunderstanging but I cannot
understand why. Thanks so much for helping!!
Delphine
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