10 research outputs found
Ophthalmic statistics note 8: missing data - exploring the unknown
Medical research is conducted to answer uncertainties and to identify effective treatments for patients. Different questions are best addressed by different types of study design—but the randomised, controlled clinical trial is typically viewed as the gold standard, providing a very high level of evidence, when examining efficacy. While clinical trial methodology has advanced considerably with clear guidance provided as to how to avoid sources of bias, even the most robustly designed study can succumb to missing data. In this statistics note, we discuss strategies for dealing with missing data but what we hope emerges is a very clear message that there is no ideal solution to missing data and prevention is the best strategy
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Ophthalmic statistics note 5: diagnostic tests-sensitivity and specificity
This is the fifth statistics note produced by the Ophthalmic Statistics Group (OSG) which is designed to be a simple guide to ophthalmic researchers on a statistical issue with an applied ophthalmic example. The OSG is a collaborative group of statisticians who have come together with a desire to raise the statistical standards of ophthalmic researcher by increasing statistical awareness of common issues
Ophthalmic statistics note 13: method agreement studies in ophthalmology-please don't carry on correlating…
Vom arbeitenden Körper zur Arbeit am Körper. Körperlichkeit alter Menschen in der zweiten Hälfte des 20. Jahrhunderts
INTRODUCTION/SCENARIO
Distributions of measured data are often well modelled by known probability distributions, which provide a useful description of their underlying properties such as location (average), spread (variation) and shape.
Statisticians use probability distributions to interpret and attribute meaning, draw conclusions and answer research questions using the measurements or data that
researchers gather during their studies. Different types of data follow different probability distributions, and these distributions are characterised by certain features
called parameters. Even the most statistically averse of researchers is likely to have heard of the normal distribution, which is often used to approximate the distribution of continuous or measurement data such as intraocular pressure, central retinal thickness and
degree of proptosis. The normal distribution follows a ‘bell-shaped curve’ (although with a rim stretching to ±infinity) the shape of which is specified by the mean and SD, with different values of each, giving rise to
different bell-shaped curves (see figure 1).
Other distributions such as the binomial and Poisson probability distributions are less commonly reported in ophthalmic research and are characterised by different
parameters. The binomial distribution is used for dichotomous data and is characterised by the probability of success, that is, the number of ‘successes’ out of a total number of observed events, for example, the proportion of graft transplants that fail within 6 months of transplantation. The Poisson distribution is used for counts data and is characterised by the mean number of
events, for example, endophthalmitis rates.
The assumption that the observed data follow such probability distributions allows a statistician to apply appropriate statistical tests, which are known as parametric tests. The normal distribution is a powerful tool provided the data plausibly arise from that
distribution or can be made to reasonably
approximate this following a suitable transformation
such as by taking natural logarithms to reduce asymmetry. The normal distribution also serves as an approximating
distribution to the Poisson or binomial distribution
under certain circumstances or can be used for large samples to approximate the distribution of the sample mean via the central limit theorem. Tests based on the
normal distribution are therefore extremely useful and form the basis of many analyses, the usual tests being z tests or t tests, which rely on approximate normality or normality, respectively.1 If we can assume a normal distribution, then we expect 95% of values to lie within 1.96 SDs of the mean.
Parametric tests make assumptions about the distribution of the data and sometimes it may be impossible to assess these assumptions, perhaps because the sample size is
small or because that data do not follow any of the more common probability distributions. Alternatively, we may be interested in making inferences about medians rather than means or about ordinal or ranked data. In such circumstances, statisticians may adopt an alternative class of statistical tests, which are known as non-parametric or distribution-free methods. These methods work by ranking the data in numerical order and analysing
these ranks rather than the actual measurements
observed. Two of the most well-known non-parametric methods are the Mann–Whitney test (or U test) and the Wilcoxon matched-pairs signed-rank test, which are
suitable for data from two unpaired samples or two paired samples, respectively.2 3
The Wilcoxon matched-pairs signed-rank test calculates the differences between each matched pair in the two samples and replaces their absolute values with their
ordered ranks (1, 2, 3, etc), ignoring zeros. Under the null hypothesis of no difference between samples, the sum of the positive and negative ranks should be similar. The
test statistic is usually taken to be the smaller of the two sums, and exact p values can be found using statistical software or by comparison with statistical tables.
The Mann–Whitney U test effectively considers all pairs of observations from two independent samples and calculates the number of pairs for which an observation
in one sample is preceded by an observation from the other. Again, the U statistic can be calculated from the
summed ranks within each sample, found by ordering the pooled observations.
Such tests depend only on the rank ordering of the observed values and not on any assumptions about their underlying distributions, so that there are no associated parameters to be estimated, and in that sense such methods are considered non-parametric or distribution-free. These are easily implemented in standard statistical software packages such as R, Stata, SAS or SPSS
Ophthalmic statistics note 14: method agreement studies in ophthalmology: the intraclass correlation coefficient?
Ophthalmic statistics note 8: missing data-exploring the unknown
Medical research is conducted to answer uncertainties and to identify effective treatments for patients. Different questions are best addressed by different types of study design—but the randomised, controlled clinical trial is typically viewed as the gold standard, providing a very high level of evidence, when examining efficacy.1 While clinical trial methodology has advanced considerably with clear guidance provided as to how to avoid sources of bias, even the most robustly designed study can succumb to missing data.2 ,3 In this statistics note, we discuss strategies for dealing with missing data but what we hope emerges is a very clear message that there is no ideal solution to missing data and prevention is the best strategy
Why is the General Ophthalmic Services (GOS) Contract that underpins primary eye care in the UK contrary to the public health interest?
The model for delivery of primary eye care in Europe varies from country to country with differing reliance on ophthalmologists, optometrists and dispensing opticians. Comparative analysis of models has tended to focus on interprofessional working arrangements, training and regulatory issues, rather than on whether a particular model is effective for delivering public health goals for that country. National Health Service (NHS) primary eye care services in the UK are predominantly provided under a General Ophthalmic Services (GOS) Contract between the NHS and practice owners (Contractors). Over two-thirds of sight tests conducted in England, Wales and Northern Ireland and all in Scotland are performed under a GOS Contract, however many people entitled to a GOS sight test do not take up their entitlement. The fee paid for sight tests conducted under a GOS Contract in England, Wales and Northern Ireland does not cover the full cost of conducting the examination. The shortfall must be made up through profits of sale of optical appliances but this business model can be a deterrent to establishing practices within socioeconomically deprived communities, and can also be a barrier to uptake of sight tests, even though many people are entitled to a NHS optical voucher towards the cost of spectacles or contact lenses. This paper critiques the GOS Contracts within the UK. We argue that aspects of the way the GOS Contract is implemented are contrary to the public health interest and that different approaches are needed to address eye health inequalities and to reduce preventable sight loss