We analyze first-year data of WMAP to determine the significance of asymmetry in summed power between arbitrarily defined opposite hemispheres .
We perform this analysis on maps that we create ourselves from the time-ordered data , using software developed independently of the WMAP team .
We find that over the multipole range l = [ 2,64 ] , the significance of asymmetry is \sim 10 ^ { -4 } , a value insensitive to both frequency and power spectrum .
We determine the smallest multipole ranges exhibiting significant asymmetry , and find twelve , including l = [ 2,3 ] and [ 6,7 ] , for which the significance \rightarrow 0 .
Examination of the twelve ranges indicates both an improbable association between the direction of maximum significance and the ecliptic plane ( significance \sim 0.01 ) , and that contours of least significance follow great circles inclined relative to the ecliptic at the largest scales .
The great circle for l = [ 2,3 ] passes over previously reported preferred axes and is insensitive to frequency , while the great circle for l = [ 6,7 ] is aligned with the ecliptic poles .
We examine how changing map-making parameters , e.g. , foreground masking , affects asymmetry .
Only one change appreciably reduces asymmetry : asymmetry at large scales ( l \leq 7 ) is rendered insignificant if the magnitude of the WMAP dipole vector ( 368.11 km s ^ { -1 } ) is increased by \approx 1-3 \sigma ( \approx 2-6 km s ^ { -1 } ) .
While confirmation of this result requires the recalibration of the time-ordered data , such a systematic change would be consistent with observations of frequency-independent asymmetry .
We conclude that the use of an incorrect dipole vector , in combination with a systematic or foreground process associated with the ecliptic , may help to explain the observed power asymmetry .