Evidence Based Medicine
Statistics
Regarding forest plots in meta-analysis, the midpoint of the square represents:
Answer:
The right-hand column is a plot of the measure of effect size, for example an odds ratio (OR), for each of these studies, represented by a square:- The midpoint of the square represents the effect size.
- The width of the horizontal lines extending from the square represents the 95% confidence interval for this effect size.
- The area of each square is proportional to the weight given to that study in the meta-analysis (studies with larger sample sizes, and more effect sizes are given more weight).
Forest Plots
Evidence Based Medicine / Statistics
Last Updated: 29th July 2024
The results of a meta-analysis are presented as a forest plot of pooled results, which provides graphical representation of the strength of evidence of the constituent trials.
Heterogeneity
Forest plots provides visual evidence of heterogeneity if present. Heterogeneity occurs when there is more variation between studies than would be expected by chance alone, even after allowing for random variation.
Heterogeneity is indicated if the confidence interval of a study does not overlap with any of the confidence intervals of the other studies. If the horizontal lines all overlap to some extent the trials are homogenous, where the observed differences are due to random variation.
Interpretation
The left-hand column lists the names of the studies, commonly in chronological order from the top downwards.
The right-hand column is a plot of the measure of effect size, for example an odds ratio (OR), for each of these studies, represented by a square:
- The midpoint of the square represents the effect size.
- The width of the horizontal lines extending from the square represents the 95% confidence interval for this effect size.
- The area of each square is proportional to the weight given to that study in the meta-analysis (studies with larger sample sizes, and more effect sizes are given more weight).
The overall meta-analysed summary effect size is plotted as a diamond:
- The midpoint of the diamond represents the summary effect size.
- The lateral points represent the 95% confidence interval for this estimate.
- There may be an ascending dotted line from the upper point of the diamond.
There is a vertical line of no effect which intersects the horizontal axis at the point at which there is no difference between the interventions.
- If the confidence intervals for individual studies overlap with this line, it demonstrates that at the given level of confidence the effect size does not differ from no effect for that individual study and therefore the results are not statistically significant. If the horizontal line does not cross the line, the results are statistically significant.
- The same applies for the summary measure of effect: if the points of the diamond overlap the line of no effect the summary effect size cannot be said to differ from no effect at the given level of confidence and the results are not statistically significant.
![By James Grellier (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], via Wikimedia Commons](https://mrcemsuccess.com/wp-content/uploads/2017/03/Forest-plot-1024x672.png)
Forest Plot. (Image by James Grellier (Own work) [CC BY-SA 3.0 , via Wikimedia Commons)
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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 |
