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James and Ben: my problem is that I find the explanation "IT MEANS in a very large number of repetitions of the study, 95% of all CIs obtained will contain the “true” value of the treatment effect in the population studied" incomprehensible. The fact is that I have conducted one study and computed a point estimate and its 95% CI. Now the hypo that if I repeat the study a thousand times and compute their CIs, 950 of the computed CIs will contain the population parameter provides no meaningful info to me because I do not know what those 950 CIs are (and to what degree they overlap or do not overlap with the CI at hand)...what I know is this CI that I have in front of me. What can I conclude from the interval I have? I think a good answer has to be philosophical rather than mathematical.....can we rephrase this in some other way? On Wed, Aug 26, 2015 at 4:07 PM, McCormack, James <[log in to unmask]> wrote: > Thanks Ben - glad you agree that I’m on the right track - it is amazing > how many different interpretations we have of confidence intervals - I’m > now 95% confident that what I said was correct :) > > > On Aug 26, 2015, at 1:00 PM, Djulbegovic, Benjamin < > [log in to unmask]> wrote: > > > I agree the statement: "The values in between these lines indicate the > range of values within which we are 95% certain (or confident, hence the > term confidence intervals) that the true value is likely to lie” is wrong. > > When we calculate a 95% confidence interval we do not calculate a “true” > assessment about this particular interval. We only say that if we repeat > our study thousands of times, 95% of the intervals we computed will contain > the true value from x1% to x2%. This does *not* mean that the true > percentage is between x1% and x2%. *The frequency with which this single > computed 95% CI contains the true value is either 100% or 0%. *(This is > akin to when we say a response rate is 40% - 4 out 10 patients will respond > to Rx, but in every individual patient the response will be either zero or > 100%). > > Ben Djulbegovic > > > *From:* Evidence based health (EBH) [ > mailto:[log in to unmask] > <[log in to unmask]>] *On Behalf Of *McCormack, James > *Sent:* Wednesday, August 26, 2015 12:13 PM > *To:* [log in to unmask] > *Subject:* Re: Use of software for obtaining confidence intervals > > Interesting take but I don’t think I’m doing that (or at least I didn’t > mean to). What I was trying to define was what a single confidence interval > from a single study means in the context of if one did 100 similar studies > and got 100 different point estimates and confidence intervals. > > I think the bottom line is that all you can say from a single confidence > interval is that it represents a plausible range of values for the effect. > > I hear people say it means that we are 95% confident that the result lies > within the range of a single CI. > > To quote an evidence-based text > > "The values in between these lines indicate the range of values within > which we are 95% certain (or confident, hence the term confidence > intervals) that the true value is likely to lie” > > which I think is wrong. > > Although I would love to be enlightened if I’m completely off base. > > Hope that makes sense. > > James > > > On Aug 26, 2015, at 8:10 AM, Mohammed T. Ansari <[log in to unmask]> > wrote: > > Thanks James. Just one question given your definition of 95% CI "*IT > MEANS in a very large number of repetitions of the study, **95% of all > CIs** obtained will contain the “true” value of the treatment effect in > the population studied (assuming random sampling)"*: > > - how can one use the same term to define the term one is defining (re" > your "*95% of all CIs*" to define a CI)? > > ____________________________________ > > > *Mohammed T. Ansari* > about.me/MTAnsari > > On Wed, Aug 26, 2015 at 10:25 AM, McCormack, James <[log in to unmask]> > wrote: > > Having taught EBHC for 25 years one of the things I see is that medical > graduates don’t understand what confidence intervals mean and do not mean - > I think more importantly than being able to calculate a CI they need to > understand the context and what it means to research findings. > > The way I understand confidence intervals is > > 1) IT MEANS in a very large number of repetitions of the study, 95% of all > CIs obtained will contain the “true” value of the treatment effect in the > population studied (assuming random sampling) > 2) IT DOES NOT MEAN the probability that the “true” magnitude of the > effect lies within the range observed in a given study > 3) IT DOES represent a plausible range of values for the effect – not a > probability of its magnitude > > Happy to hear if I am misinterpreting the concept > > We recently wrote an article showing how authors confuse the meaning of > confidence intervals. This group may find it of value as we provide a > number of examples (statins and mortality in primary prevention) where > authors misinterpret their results because they misinterpret what a > confidence interval means and does not mean. > > http://www.ncbi.nlm.nih.gov/pubmed/24172248 > > James McCormack > > > > On Aug 25, 2015, at 5:05 AM, Margaret MacDougall <[log in to unmask]> > wrote: > > Hello > > As a medical statistician, I have strong support for the use of confidence > intervals in the reporting of clinical research findings. However, I am > also conscious that medical graduates may lose their access to licensed > software previously used in their undergraduate learning to support such > activities. For example, both Minitab and Confidence Interval Analysis are > possible choices for non-specialists and often supported by University > licenses. However, do medical graduates have an incentive to obtain their > own licenses for such software on entering the workplace? I would be > interested to learn from medical graduates as to their personal choices in > the above context and indeed, as to whether or not such persons have found > freeware sufficiently reliable and easy to use to adopt in their own > working practices specifically for obtaining confidence intervals (not for > statistics more generally). This query is also of relevance to the > management of courses for distance learners, not all of whom may have > access to a licensed statistical package for calculating confidence > intervals but still need to perform the calculations to meet programme > requirements. > > Many thanks in advance > > Best wishes > > Margaret > > > -- ____________________________________ [image: --] Mohammed T. Ansari [image: https://]about.me/MTAnsari <https://about.me/MTAnsari?promo=email_sig>