Evidence Based Medicine
Statistics
A p value < 0.05 obtained from a study with a significance level (α) of 0.05, means all of the following, EXCEPT:
Answer:
A p value < 0.05:- is statistically significant
- means that the probability of obtaining a given result by chance is less than 1 in 20
- means the null hypothesis is rejected
- means there is evidence of an association between a variable and an outcome
Hypothesis Testing
Evidence Based Medicine / Statistics
Last Updated: 21st April 2019
Hypothesis testing involves making estimates or predictions about something unknown using some prior knowledge, thus moving from descriptive statistics to inferential statistics. It necessitates making assumptions and therefore dealing with error and significance.
Null Hypothesis
The null hypothesis assumes that there is no true difference between two groups of data and that any difference observed in the results of two groups of data is due to chance.
The alternative hypothesis assumes that there is a true difference between two groups of data, and is by default accepted if the null hypothesis is rejected.
Tests can either be one- or two- tailed, depending on the null hypothesis being tested. A one-tailed hypothesis specifies the direction of a difference or correlation (e.g. treatment A is not superior to treatment B) and can be tested with a smaller sample than a two-tailed hypothesis (treatment A is equivalent to treatment B).
P Value
To test a null hypothesis, a p value is calculated using a statistical significance test.
The p value gives the probability of observing a difference between the two groups if the null hypothesis is true. In other words, the probability that any observed difference has happened by chance and not by any true difference.
The larger the p value, the more likely it is that the data have arisen by chance alone, that the null hypothesis is true and that they do not represent a real difference between the groups.
The smaller the p value, the less likely it is that the difference has happened by chance, the greater the evidence against the null hypothesis and so the higher the statistical significance of the finding.
Statistical Significance
It is important that the null hypothesis should only be rejected when a chance error is very unlikely as we do not want to conclude that a treatment is effective when it is not. For medical interventions, it is conventional to set the significance level at 0.05 which means that the null hypothesis should be rejected in favour of the alternative hypothesis if p < 0.05.
A p value < 0.05:
- is statistically significant
- means that the probability of obtaining a given result by chance is less than 1 in 20
- means the null hypothesis is rejected
- means there is evidence of an association between a variable and an outcome
A p value > 0.05:
- is statistically non-significant
- means that the probability of obtaining a given result by chance is more than 1 in 20
- means that the null hypothesis is accepted
- means that an association between a variable and an outcome has not been proved
Clinical vs Statistical Significance
It is important to appreciate that statistical significance does not imply clinical significance. If a study is too small, the results are unlikely to be statistically significant even if the intervention actually works. Conversely a large study may find a statistically significant difference that is too small to have any clinical relevance.
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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 |
