Part 1 which appeared in the February 2012 issue introduced the concept of confidence intervals CIs for mean values This article explains how to compare the Cls of two mean scores to draw a conclusion about whether or not they are statistically different Two mean scores are said to be statistically different if their respective Cls do not overlap Overlap of the Cls suggests that the scores may represent the same true population value in other words the true difference in the mean scores may be equivalent NurseONE resources ON THIS TOPIC EBSCO MEDLINE FULL TEXT ARTICLES Hildebrandt M Vervolgyi E Bender R 2009 Calculation of NNTs in RCTS with time to event outcomes A Iterature review BMC Medical Research Methodology 9 21 Hildebrandt M Bender R Gehrmann U Biettner M 2006 Calculating confidence intervals for impact numbers 8MC Medical Research Methodology 6 32 Altman D G 1998 Confidence intervals for the number needed to treat BM Cnical Research Ed 317 7168 1309 1312 MiLIBRARY Campbell M I Machin D Watters S 2010 Medical statistics A textbook for the health sciences 4th edi Mates M Kischhoff K T Eds 2009 Research for advanced practice nurses From evidence to practice Webb C Ror 8 Eds 2007 Reviewing research evidence for nursing practice Systematic revies to zero Some researchers choose to provide the CI for the difference of two mean scores instead of providing a separate CI for each of the mean scores In that case the difference in the mean scores is said to be statistically significant if its Cl does not include zero e g if the lower limit is 10 and the upper limit is 30 If the Cl includes zero e g if the lower limit is 10 and the upper limit is 30 we conclude that the observed difference is not statistically significant To illustrate this point let s say that we want to compare the mean blood pressure BP of exercising and sedentary patients The mean BP is 120 mmHg 95 CI 110 130 mmHg for the exercising group and 140 mmHg 95 CI 120 160 mmHg for the non exercising group We notice that the mean BP values of the two groups differ by 20 mmHg and we want to determine whether this difference is statistically significant Notice that the range of values between 120 and 130 mmHg falls within the Cls for both groups i e the Cls overlap Thus we conclude that the 20 mmHg difference between the mean BP values is not statistically significant Now say that the mean BP is 120 mmHg 95 CI 110 130 mmHg for the exercising group and 140 mmHg 95 CI 136 144 mmHg for the sedentary group In this case the two Cla do not overlap none of the values within the first CI fall within the range of values of the second Cl Thus we conclude that the mean BP difference of 20 mmHg is statistically significant Remember we can use either the Cls of two mean scores or the Cl of their difference to draw conclusions about whether or not the observed difference between the scores is statistically significant