Table A-7 lists critical values for limited choices of a. Use Table A-l to add a new column in Table A-7 (down to n = 15) that represents a significance level of 0.03 in one tail or 0.06 in two tails. For any particular n we use p = 0.5, as the sign test requires the assumption that
P(positive sign) = P(negative sign) = 0.5
The probability of x or fewer like signs is the sum of the probabilities for values up to and including x.