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Evidence Based Medicine

Statistics

Question 103 of 180

In hypothesis testing, what does alpha denote:

Answer:

The probability of making a type I error is denoted by alpha (α), the predetermined significance level of the test (conventionally α = 0.05).

Type I and Type II Errors

Evidence Based Medicine / Statistics

Last Updated: 21st April 2019

Type I Error

A type I error occurs when the null hypothesis is wrongly rejected when it is actually true and we conclude that there is a difference of effect when in reality there is none (a false positive result).

The probability of making a type I error is denoted by alpha (α), the predetermined significance level of the test (conventionally α = 0.05).

  • A p value < 0.05 means that the probability of obtaining a given result by chance is less than 1 in 20. This is statistically significant and means the null hypothesis is rejected and there is evidence of an association between a variable and an outcome.
  • A p value > 0.05 is statistically non-significant and the null hypothesis is accepted. An association between a variable and an outcome has not been proved.

Type II Error

A type II error occurs when the null hypothesis is wrongly accepted when it is actually false and we conclude that there is no evidence of a difference in effect when one really exists (a false negative result).

The probability of making a type II error is denoted by beta (β).

Type II errors can be avoided at the design stage of the study by power calculations that give an indication of the minimum sample size required so that one can be reasonable likely to detect an effect of a given size.

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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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