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Archimedes
Towards evidence based medicine for paediatricians Edited by Bob Phillips Propensity scores Propensity scores are used mainly in observational studies assessing treatments as a way of balancing out measured variations in who received a treatment and who didn’t. In most observational studies, there are things which will have pushed the doc into prescribing the medicine in question, or the surgeon to take the knife to that patient and not the other one. These factors (baseline characteristics, if you’d like to be fancy) might well be linked to the outcome, as well as linked to the decision to treat. For example, if you are looking at if antihistamines are helpful in chickenpox itch, it may be that those with a greater number of spots were more likely to be prescribed chlorpheniramine, and that the spot count is also related to the amount of reported itch. What a propensity score attempts to do is to ‘balance’ the data into groups—often five of them (quintiles)—who contain equal distributions of the baseline characteristics (like spottyness, gender and number of sibs) This score can then be used to match up treated and untreated patients, or used to create five different treatment effectiveness estimates across the different strata, or adjust a regression analysis, but essentially used to make a guess at the real effectiveness of the treatment and ‘correct’ for measured biases. (You’ll note that this has no way of balancing up unmeasured biases, and probably guessed that if the measurement tools are coarse—eg, saying “severity of illness was measured as intensive care unit admission vs. not”—then there may remain unaccounted biases.) While propensity scores can help in making fairer comparisons across some observational data sets, they are not a way of avoiding randomised controlled trials. So if you want to know if you should give piriton to the itchy pox-filled children in your life, you really should be enrolling them on a proper trial. Correspondence to Dr Bob Phillips, Centre for Reviews and Dissemination, University of York, York YO10 5DD, UK; [email protected] Competing interests None. Provenance and peer review Commissioned; internally peer reviewed. Received 31 October 2013 Accepted 1 November 2013
▸ http://dx.doi.org/10.1136/archdischild-2013-305591 Arch Dis Child 2014;99:180. doi:10.1136/archdischild-2013-305593
180
Arch Dis Child February 2014 Vol 99 No 2
Downloaded from http://adc.bmj.com/ on March 16, 2015 - Published by group.bmj.com
Towards evidence based medicine for paediatricians Bob Phillips Arch Dis Child 2014 99: 180
doi: 10.1136/archdischild-2013-305593 Updated information and services can be found at: http://adc.bmj.com/content/99/2/180.2
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To request permissions go to: http://group.bmj.com/group/rights-licensing/permissions To order reprints go to: http://journals.bmj.com/cgi/reprintform To subscribe to BMJ go to: http://group.bmj.com/subscribe/
Archimedes
Towards evidence based medicine for paediatricians Edited by Bob Phillips Propensity scores Propensity scores are used mainly in observational studies assessing treatments as a way of balancing out measured variations in who received a treatment and who didn’t. In most observational studies, there are things which will have pushed the doc into prescribing the medicine in question, or the surgeon to take the knife to that patient and not the other one. These factors (baseline characteristics, if you’d like to be fancy) might well be linked to the outcome, as well as linked to the decision to treat. For example, if you are looking at if antihistamines are helpful in chickenpox itch, it may be that those with a greater number of spots were more likely to be prescribed chlorpheniramine, and that the spot count is also related to the amount of reported itch. What a propensity score attempts to do is to ‘balance’ the data into groups—often five of them (quintiles)—who contain equal distributions of the baseline characteristics (like spottyness, gender and number of sibs) This score can then be used to match up treated and untreated patients, or used to create five different treatment effectiveness estimates across the different strata, or adjust a regression analysis, but essentially used to make a guess at the real effectiveness of the treatment and ‘correct’ for measured biases. (You’ll note that this has no way of balancing up unmeasured biases, and probably guessed that if the measurement tools are coarse—eg, saying “severity of illness was measured as intensive care unit admission vs. not”—then there may remain unaccounted biases.) While propensity scores can help in making fairer comparisons across some observational data sets, they are not a way of avoiding randomised controlled trials. So if you want to know if you should give piriton to the itchy pox-filled children in your life, you really should be enrolling them on a proper trial. Correspondence to Dr Bob Phillips, Centre for Reviews and Dissemination, University of York, York YO10 5DD, UK; [email protected] Competing interests None. Provenance and peer review Commissioned; internally peer reviewed. Received 31 October 2013 Accepted 1 November 2013
▸ http://dx.doi.org/10.1136/archdischild-2013-305591 Arch Dis Child 2014;99:180. doi:10.1136/archdischild-2013-305593
180
Arch Dis Child February 2014 Vol 99 No 2
Downloaded from http://adc.bmj.com/ on March 16, 2015 - Published by group.bmj.com
Towards evidence based medicine for paediatricians Bob Phillips Arch Dis Child 2014 99: 180
doi: 10.1136/archdischild-2013-305593 Updated information and services can be found at: http://adc.bmj.com/content/99/2/180.2
These include:
Email alerting service
Topic Collections
Receive free email alerts when new articles cite this article. Sign up in the box at the top right corner of the online article.
Articles on similar topics can be found in the following collections ADC Archimedes (203) Competing interests (ethics) (145) Clinical trials (epidemiology) (427) Journalology (195)
Notes
To request permissions go to: http://group.bmj.com/group/rights-licensing/permissions To order reprints go to: http://journals.bmj.com/cgi/reprintform To subscribe to BMJ go to: http://group.bmj.com/subscribe/
