values DV, has a normal distribution with mean 0 and SD=sqrt(SD2^2 + SD2^2).
So in this case DV=(55.7-60.48) = - 4.78, and SD = sqrt ( (3.42/2)^2 + (3.56)^2) = 2.47
As |DV|<1.96 SD, you can say that the data does not allow you to reject the hypothesis
that cleaning has had no effect in this case at 95% level of confidence (if the hyposthesis
were true the probability of rejection with this test < 5%)