Answer
$T\lt T_α$: null hypothesis is rejected.
There is enough evidence to conclude that the median hemoglobin level at lift-off minus 3 days is less than the median hemoglobin level upon return.
Work Step by Step
$H_0:M_D=0$ versus $M_D\lt0$
Let the "H-L3" values to be the X and the "H-R0" values to be the Y.
$D_i=X_i-Y_i~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Rank$
$D_1=X_1-Y_1=15.2-15.8=-0.6~~~~~~~~~~~~-2$
$D_2=X_2-Y_2=16.1-16.5=-0.4~~~~~~~~~~~~-1$
$D_3=X_3-Y_3=15.3-16.7=-1.4~~~~~~~~~~~~-8$
$D_4=X_4-Y_4=16.4-15.7=0.7~~~~~~~~~~~~~~~+3.5$
$D_5=X_5-Y_5=15.7-16.9=-1.2~~~~~~~~~~~~-7$
$D_6=X_6-Y_6=14.7-13.1=1.6~~~~~~~~~~~~~~~+9$
$D_7=X_7-Y_7=14.3-16.4=-2.1~~~~~~~~~~~~-11$
$D_8=X_8-Y_8=14.5-16.5=-2~~~~~~~~~~~~~~~-10$
$D_9=X_9-Y_9=15.2-16.0=-0.8~~~~~~~~~~~~-5$
$D_{10}=X_{10}-Y_{10}=16.1-16.8=-0.7~~~~~~~~-3.5$
$D_{11}=X_{11}-Y_{11}=15.1-17.6=-2.5~~~~~~~~-12$
$D_{12}=X_{12}-Y_{12}=15.8-16.9=-1.1~~~~~~~~-6$
$n=12$
Left-tailed test.
Test statistic: $T=T_+=3.5+9=12.5$
Critical value: $T_α=17$
(According to table XII, for $n=12$ and $α=0.05$)
Since $T\lt T_α$, we reject the null hypothesis.
