Quality Control

mean

the average of a group of data points

e.g. (120, 115, 120, 109, 111)

575/5 = 115

mode

the most commonly occurring value in a group of data points

e.g. (120, 115, 120, 109, 111)

120 (occurs twice)

median

the middle value in a group of data points

e.g. (120, 115, 120, 109, 111)

109, 111, 115, 120, 120

middle = 115

Gaussian distribution (normal distribution)

a theoretical distribution in which the mean, median and mode are equal

standard deviation

the dispersion of data points around the mean

standard deviation formula (provided)

standard deviation steps

1. calculate the mean

2. subtract the mean from each value

3. square the difference for each value

4. add up the squared differences

5. divide by (n-1)

6. take the square root

n

the number of data points

Percent falling within 2 SD

95%

acceptable variation in QC data

+/- 2 standard deviations from the mean

95% confidence

95% of the values (19 results out of 20) should be within the allowable variation of +/- 2SD according to the Gaussian distribution

5% error

5% of the values (1 result out of 20) may be outside the allowable variation of +/- 2SD due to chance

acceptable QC range of: (mean = 115, SD = 5)

= mean +/- 2SD

= 115 +/- 10

= 105-125

coefficient of variation

a measure of the distribution of a data set in relation to its mean

helps to compare data sets when their mean is not the same

A lower CV value has

higher reliability

coefficient of variation formula

CV of a data set with: (mean = 115, SD = 5)

= SD / mean (x 100%)

= 5 / 115 (x 100%)

= 4 %

sensitivity

ability of a test to detect a given disease or condition when present

able to detect small quantities or changes in an analyte

specificity

ability of a test to correctly identify individuals without a disease or condition

ability to only detect desired analyte (cross-reactivity with other substances)

sensitivity formula

specificity formula

positive predictive value

probability of the individual having the disease if the result is positive (abnormal)

negative predictive value

probability of the individual not having the disease if the result is negative (normal)

positive predictive value formula

negative predictive value formula