Post on 06-Jan-2016
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Prof. of Clinical Chemistry, Mansoura University
Statistical procedures
Statistics defined as the science of:
• Gathering data• Analyzing data• Interpreting data • Presenting data
Discriptive statistics
Used to summarize the important feature of a group of data
Frequency histogram (diagram):
Repeated measurements The number of frequency of each result
on y – axis The value of the result on x-axis
If this histogram is bell shape (Gaussian Gaussian distributiondistribution) date analysed by standard
(parametricparametric) statistical tests (small departure don’t affect result).
If the data deviate greatly from Gaussian distribution, it will be analyzed with
distribution free (non parametricnon parametric) statistical test.
Skewness:
The most common deviation from Gaussian distribution
It is the presence of increased numbers of observation in one of the tails of the
distribution
It is used to calculate the statistical limit for the mean
It is the average error encountered if the sample mean was used to
estimate the population mean .SEM decreases as the sample size
increases The mean of large sample is likely to be closer to the true mean than the
mean of a small sample
The introduction of a new test illustrates the extensive use of
statistics in the laboratory.The test should be introduced only
ofter reviewing the data that document its usefulness in
diagnosis or monitoring a disease state.
If there are several methods to do this test chose the optimally acurate
and precise method accuracy and precision must be continualy
assessed to ensure reliable analyses
Descriptive statistics of groups of paired observations
This is the comparison – of – methods experiment in which the specirmens are measured by both
the new method and the old or comparative method.
Old method values are plotted on the x-axis .new method values are
plotted on the y-axis.
Linear regression analysis
Y = m x + y o The slop = m
Yo = y at x = O If there is perfect agreement
between the two methods , each value measused by the test method
will be equal to that measured by the comparative one
So y = x with m = 1 and yo = O
To distinguish between to variables:
Calibration involves measurment of the instrument response Y (absorbance) to
special samples called calibrators whose concentration X are known.
The calibration procedure is performed periodically to adjust for system drift.
Calibration:
Two kinds of errors are measured in comparison of method experiments (random and systemic error):
present in all measurements (if repeated)
due to chance can be positive or negative
The measure of dispersion sy/x estimate this error
Errors account for the difference between the test and comparative-method results.
Random error:Random error:
Systematic error (S.E): Systematic error (S.E):
Influences consistently in one direction
Should not be present in a method The slop and Y intercept can
measure systematic E May be constant regardless of the
concentration (constant S.E) Or proportional to analyte
concentration (proportional S.E)
Total analysis error (Xd) = XTotal analysis error (Xd) = Xtesttest – C – Ccompcomp..
(at least 40 specimen)(at least 40 specimen)Comparative method = reference method
e.g. chromatography.
Scatterplot of the individual measurements XJaffe against X HPLC, with least squares line (solid line) and line of identity (dotted line).
Inferential statistics
To compare the means and SDS of two groups of data.
T – test: used to determine if there is statistically significant difference between
means.
F – test :
• Between standard deviations.• Both have limited usefulness in
method evaluation Sig < 0.05 = the probabilty of the difference
between the two groups of data dlue to chance is less than 0.0
Factors necessary : Respect to age , sex and genetic
socioeconomic factors Physiologic and environmental
conditionsCriteria of excluding or including
individuals Specimen collection procedure
The analytical methods Normal value and normal range is that
corrospond to the health associated reference interval ( central 95%)
Reference entervals = Normal Range Reference entervals = Normal Range
Test performance characteristics:
Linear range: