Nav: Home

Zinc effects on common cold duration illustrate problems of routine statistical analyses

May 18, 2017

Two randomized trials that examined the effects of zinc lozenges for the duration of common cold symptoms found that colds were shortened on average by 4.0 days and by 1.77 days. However, the shortest colds in the placebo groups of both studies lasted for only 2 days.

According to a paper published in BMC Medical Research Methodology, when the effect estimates (average shortening of colds in days) were applied to the duration of colds in the respective placebo groups, negative and zero cold durations were predicted, i.e. impossible and unrealistic predictions. In contrast, when the corresponding percentage effect estimates, 43% and 25%, respectively, were applied to the placebo group common cold durations, no impossible durations were predicted.

In the standard statistical analysis of continuous outcomes such as common cold duration, two scales are widely used for measuring the treatment effect. The absolute scale indicates the calculation of effect as the number of days by which the colds were shortened. The second scale widely used has the standard deviation as the unit of the scale, which leads to the standardized mean difference scale (SMD scale). Both of these approaches are available as options in popular meta-analysis software such as the RevMan program of the Cochrane collaboration.

If the absolute effects of a 4.0 day and a 1.77 day reductions in common cold duration are assumed to be uniform effects of taking zinc lozenges, then they should be valid for both short and long lasting colds. However, Dr. Harri Hemilä from the University of Helsinki, Finland, showed that the reduction in the duration of short colds was less than the average, whereas the reduction in the duration of long colds was much more than the average in the two studies that were analyzed. The implication of this finding is that the use of 4.0 and 1.77 day zinc effects for all colds is misleading and illogical. In contrast, the percentage effects of 43% and 25% realistically described the effects of zinc lozenges on both short duration and long duration colds.

The SMD scale is quite confusing for ordinary readers of research reports. For example, the 2011 Cochrane review on zinc and the common cold stated in its abstract that the "intake of zinc is associated with a significant reduction in the duration (standardised mean difference (SMD) -0.97)". Research reporting should always show the unit of measurement, and this sentence should have been written more accurately as: "zinc shortened the duration of colds by 0.97 standard deviation units". However, such accurate reporting would reveal the main problem of using the SMD scale: What does the "standard deviation unit" of common cold duration mean in practical terms? Most physicians and patients can easily understand whether 42% or 25% is a small or a large effect, but few of them can readily comprehend whether an effect of 0.97 standard deviation units is small or large. In this respect, the percentage effect scale is much more tangible and far superior in the communication of findings to physicians, and between physicians and patients, since the population as a whole are familiar with percentage effects.

Although the goal of the study was not to estimate the overall effect of zinc lozenges on common cold duration, the findings of the two trials analyzed are consistent with the percentage effects of zinc lozenges observed in four other trials on zinc lozenges.

Dr. Hemilä states that "The common cold is by itself a clinically relevant topic and the percentage scale leads to much more effective information about the size of the treatment effect than the absolute scale and the SMD scale. It seems evident that the percentage scale should be used in the analysis of many other continuous study outcomes, such as the duration of other diseases, and the duration of hospital stay and so on." Dr. Hemilä is concerned about certain situations when the Cochrane Collaboration has not allowed the usage of the percentage effect scale in the analysis of continuous outcomes even though it is much more informative than the absolute and SMD scales.

University of Helsinki

Related Zinc Articles:

A nanoscale laser made of gold and zinc oxide
Tiny particles composed of metals and semiconductors could serve as light sources in components of future optical computers, as they are able to precisely localize and extremely amplify incident laser light.
Zinc lozenges did not shorten the duration of colds
Administration of zinc acetate lozenges to common cold patients did not shorten colds in a randomized trial published in BMJ Open.
Dietary zinc protects against Streptococcus pneumoniae infection
Researchers have uncovered a crucial link between dietary zinc intake and protection against Streptococcus pneumoniae, the primary bacterial cause of pneumonia.
Zinc could help as non-antibiotic treatment for UTIs
New details about the role of zinc in our immune system could help the development of new non-antibiotic treatment strategies for bacterial diseases, such as urinary tract infections (UTIs).
Zinc deficiency may play a role in high blood pressure
Lower-than-normal zinc levels may contribute to high blood pressure (hypertension) by altering the way the kidneys handle sodium.
Genetic polymorphisms and zinc status
Zinc is an essential component for all living organisms, representing the second most abundant trace element, after iron.
Autism is associated with zinc deficiency in early development -- now a study links the two
Autism has been associated with zinc deficiency in infancy. While it is not yet known whether zinc deficiency in early development causes autism, scientists have now found a mechanistic link.
Can chocolate, tea, coffee and zinc help make you more healthy?
Ageing and a low life expectancy are caused, at least partly, by oxidative stress.
Zinc oxide nanoparticles: Therapeutic benefits and toxicological hazards
Despite the widespread application of zinc oxide nanoparticles in biomedicine, their use is still a controversial issue.
Preconception zinc deficiency could spell bad news for fertility
The availability of micronutrients in the ovarian environment and their influence on the development, viability and quality of egg cells is the focus of a growing area of research.
More Zinc News and Zinc Current Events

Trending Science News

Current Coronavirus (COVID-19) News

Top Science Podcasts

We have hand picked the top science podcasts of 2020.
Now Playing: TED Radio Hour

Listen Again: Reinvention
Change is hard, but it's also an opportunity to discover and reimagine what you thought you knew. From our economy, to music, to even ourselves–this hour TED speakers explore the power of reinvention. Guests include OK Go lead singer Damian Kulash Jr., former college gymnastics coach Valorie Kondos Field, Stockton Mayor Michael Tubbs, and entrepreneur Nick Hanauer.
Now Playing: Science for the People

#562 Superbug to Bedside
By now we're all good and scared about antibiotic resistance, one of the many things coming to get us all. But there's good news, sort of. News antibiotics are coming out! How do they get tested? What does that kind of a trial look like and how does it happen? Host Bethany Brookeshire talks with Matt McCarthy, author of "Superbugs: The Race to Stop an Epidemic", about the ins and outs of testing a new antibiotic in the hospital.
Now Playing: Radiolab

Dispatch 6: Strange Times
Covid has disrupted the most basic routines of our days and nights. But in the middle of a conversation about how to fight the virus, we find a place impervious to the stalled plans and frenetic demands of the outside world. It's a very different kind of front line, where urgent work means moving slow, and time is marked out in tiny pre-planned steps. Then, on a walk through the woods, we consider how the tempo of our lives affects our minds and discover how the beats of biology shape our bodies. This episode was produced with help from Molly Webster and Tracie Hunte. Support Radiolab today at