An office manager records the following number of phone calls over 33 days: 65 62 55 62 79 59 63 73 71 66 94 73 95 66 80 94 79 51 73 72 56 76 70 61 71 81 88 51 70 90 73 85 69 What is the standard deviation for this data set? (round to 4 decimal places)