Q8 Homework -- Paired-Samples t-test 

4.   A group of 40 city parks were using what was hoped to be a crime reducing strategy in effort to combat assaults. They measured assaults and treated each park with camera monitoring devices. They tested again after the treatment. The mean before treatment (pre) was = 39.2.  The parks were tested six months later to ascertain if the cameras had the desired effect. The mean after treatment (post) was = 35.83 (SEdif = 1.420). The assaults were expected to decrease after a six-month period.

Answer   Difference in assualts pre post         

  sig

Facts    

N 40.00  

Mean Before  39.20 

Mean After   35.83 

SE dif    1.420 

df   39  

tcrit = df 50    2.009 

t obt            2.38  

n2 (computed) 0.13