When the results of all available clinical trials are combined in a Systematic Review (for example in a Cochrane review) care is still required in the interpretation of the results from each individual trial, and the main focus is on the pooled result giving the average from all the trials. The results are often displayed in a forest plot. The result of each trial is represented by a rectangle (which is larger for the bigger trials) and the horizontal lines indicate the 95% confidence interval of each trial. The diamond at the bottom is the pooled result and its confidence interval is the width of the diamond.
In a Cochrane review comparing Holding Chambers (Spacers) with nebulisers to deliver salbutamol in acute asthma, hospital admissions for acute asthma were rare in each trial. This means that the uncertainty of the individual trials is seen in wide confidence intervals but when these are pooled together the uncertainty shrinks to a much narrower estimate. The pooled odds ratio of one indicates no difference shown between delivery methods for beta-agonists in acute asthma as far as admission rates are concerned, but the estimate is still imprecise and compatible with both a halving and a doubling of the odds of being admitted to hospital. So we have to say that we do not know whether there is a difference in the rate of admissions between the two delivery methods.