Standard Deviation Calculations

Standard deviation calculations Clinical chemistry

• Given: +/-2SD= 0.5-1.3 mg/dl • Need to find +/-1SD and +/-3SD?????? • We need mean and SD before we can calculate these values • Let’s calculate mean first: • Two steps • Take the given 2 SD values and add them • Then divide that value by 2

• 0.5+1.3+1.8 • MEAN=0.9

1.8/2=0.9

• Second we need to find the SD: • Two steps: • Subtract the two given values of 2SD • Then divide that value by 4 • 1.3-0.5=0.8 • SD=0.2

0.8/4=0.2

• Now we can go on and find the +/-SD • +1sd= mean+1(sd) 0.9+1 (0.2)=1.1

• -1sd= 0.9-1(0.2)=0.7 • +/-1sd=0.7-1.1

•

now the +/-3SD • +3sd=0.9+3(0.2)=1.5 • -3sd=0.9-3(0.2)=0.3 • +/-3sd=0.3-1.5