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Evidence Based Medicine

Statistics

Question 114 of 180

Regarding measures of central tendency, which of the following statements is CORRECT:

Answer:

The median is the middle value of a data set that has been placed in numerical order, and the point at which half of the observations lie above it and half below it. The median is not distorted by outliers or skewed data. The mode is the most commonly recurring number. The median is similar to the mean if the data is symmetrical, less than the mean if the data are skewed to the right and greater than the mean if the data are skewed to the left.

Measures of Central Tendency

There are three measures of central tendency: mean, median and mode.

  • Normal distribution (no skew): Mean = median = mode
  • Positive skew (skewed to the right): Mean > median > mode
  • Negative skew (skewed to the left): Mean < median < mode

Mean

The mean is the arithmetic average, calculated by summing all of the values of a data set and dividing this sum by the number of observations in the data set.

The mean is used in normally distributed data as it uses all of the data set and roughly reflects the sampling distribution. However the mean is distorted by outliers and skewed data, where the median should be used instead.

Median

The median is the middle value of a data set that has been placed in numerical order starting with the smallest value and ending with the largest value. With an even number of data values, the median is the average of the two values that lie on either side of the midline. It is the point which has half of the observations above it and half of the observations below it.

The median is similar to the mean if the data is symmetrical, less than the mean if the data are skewed to the right and greater than the mean if the data are skewed to the left.

The median is not distorted by outliers or skewed data but ignores most of the data values and so it can be shown to be less efficient than the mean.

Mode

The mode is the value that is observed most frequently in a data set.

The mode is rarely used as a summary measure. It is easily determined for categorical data but ignores most of the information and does not reflect sampling distribution.

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  • Biochemistry
  • Blood Gases
  • Haematology
Biochemistry Normal Value
Sodium 135 – 145 mmol/l
Potassium 3.0 – 4.5 mmol/l
Urea 2.5 – 7.5 mmol/l
Glucose 3.5 – 5.0 mmol/l
Creatinine 35 – 135 μmol/l
Alanine Aminotransferase (ALT) 5 – 35 U/l
Gamma-glutamyl Transferase (GGT) < 65 U/l
Alkaline Phosphatase (ALP) 30 – 135 U/l
Aspartate Aminotransferase (AST) < 40 U/l
Total Protein 60 – 80 g/l
Albumin 35 – 50 g/l
Globulin 2.4 – 3.5 g/dl
Amylase < 70 U/l
Total Bilirubin 3 – 17 μmol/l
Calcium 2.1 – 2.5 mmol/l
Chloride 95 – 105 mmol/l
Phosphate 0.8 – 1.4 mmol/l
Haematology Normal Value
Haemoglobin 11.5 – 16.6 g/dl
White Blood Cells 4.0 – 11.0 x 109/l
Platelets 150 – 450 x 109/l
MCV 80 – 96 fl
MCHC 32 – 36 g/dl
Neutrophils 2.0 – 7.5 x 109/l
Lymphocytes 1.5 – 4.0 x 109/l
Monocytes 0.3 – 1.0 x 109/l
Eosinophils 0.1 – 0.5 x 109/l
Basophils < 0.2 x 109/l
Reticulocytes < 2%
Haematocrit 0.35 – 0.49
Red Cell Distribution Width 11 – 15%
Blood Gases Normal Value
pH 7.35 – 7.45
pO2 11 – 14 kPa
pCO2 4.5 – 6.0 kPa
Base Excess -2 – +2 mmol/l
Bicarbonate 24 – 30 mmol/l
Lactate < 2 mmol/l

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