Category Archives: Part A

Epidemiology – Numbers Needed to Treat (NNT)


NNT = the number of patients that need to be treated in order for 1 extra patient to benefit

Alternatives to NNT include:

  • Numbers Needed to Screen (NNS = No. needed to be screened for 1 to benefit)
  • Numbers Needed to Harm (NNH = No. needed to be exposed to a risk factor for 1 to be harmed)


  • NNT = 1/ARR
  • Absolute Risk Reduction (ARR) is calculated by the difference between the rate of event in controls and the rate of event in cases = (a/a+c) – (b/b+d)
  • NNTs should always be reported with 95% Confidence Intervals for interpretation


  • The lower the NNT the better.
  • E.g. Drug FAB helps prevent strokes and has an NNT of 1.  By treating Bob with FAB this should prevent him having a stroke.  On the otherhand, drug BAD has an NNT of 50, so you would have to treat 50 Bobs in order to prevent one stroke.
  • If the treatment or exposure if harmful (i.e. result is a negative number) the omit the sign and measure is renamed as NNH


  • Useful to communicate benefit and harm – easy to understand (risk communication)
  • NNTs can be used either for summarising the results of trials
  • A clinically useful measure of the relative benefit of an active treatment over a control (better than RR or OR)
  • Takes into account the frequency of the outcome – thus reflects the ublic health impact of the intervention


  • Cannot be used for performing a meta-analysis. Pooled NNTs derived from meta-analyses can be seriously misleading because the baseline risk often varies appreciably between the trials
  • Do not compare NNTs for different therapies *unless* the baseline risks of the disease are similar…

For further info, check out


Epidemiology – Odds Ratio (OR)


The Odds Ratio is a measure of association which compares the odds of disease of those exposed to the odds of disease those unexposed.


  • OR = (odds of disease in exposed) / (odds of disease in the non-exposed)


I often think food poisoning is a good scenario to consider when interpretting ORs:  Imagine a group of 20 friends went out to the pub – the next day a 7 were ill.  They suspect that it may have been something they ate, maybe the fish casserole… here are the numbers:

(not ill)
(ate fish)
5 3 8
(didn’t eat fish)
2 10 12
7 13 20
  • Odds of exposure in cases = a/c = 5/2 = 2.5
  • Odds of exposure in controls = b/d = 3/10 = 0.3
  • Odds Ratio = (a/c) / (b/d) = 2.5/0.3 = 8.33

Interpretation: What does this mean?

  • OR of 1 would suggests that there is no difference between the groups; i.e. there would be no association between the suggested exposure (fish) and the outcome (being ill)
  • OR of > 1 suggests that the odds of exposure are positively associated with the adverse outcome compared to the odds of not being exposed
  • OR of < 1 suggests that the odds of exposure are negatively associated with the adverse outcomes compared to the odds of not being exposed.  Potentially, there could be a protective effect

In the example above, we can conclude that those who ate the fish casserole (exposure) were 8.3 times more likely (OR = 8.3) to be ill (outcome), compared to those who did not eat the fish casserole.  Of course this is an entirely ficticious example, and I have nothing against fish


  • Appropriate to analyse associations between groups from case-control and prevalent (or cross-sectional) data.
  • For rare diseases (or diseases with long latency periods) the OR can be an approximate measure to the RR (relative risk)
  • Doesn’t require denominator (i.e. total number in population) unlike measuring risk
  • Good method to estimate the strength of an association between exposures and outcomes


  • Association does not infer causation! *epidemiology golden rule*

Exam Techniques

This week I attended: “Part A Revision Seminar: How to Answer Questions!”

This was a useful session which (thankfully) built up my confidence and also provided some useful practical tips, which I thought might be helpful to share.

(I would like to acknowledge and thank Kirsteen Macleod (ST5) for sharing her wisdom and collated advice – the following information have been taken from her presentation)

ARGH I’ve got to answer a question – how do I start?

  • Identify what aspect of the curriculum the question covers
  • Do a brain dump of all ideas/knowledge/critiques… e.g. spider diagram
  • Think laterally – why is this important to public health? Have wider determinents been considered?
  • Identify an appropriate structure (something logical, e.g. framework)
  • Write a structured answer with bullet points, neat handwriting and headings
  • Use examples to illustrate your answer

What (other than knowledge) is essential to PASS the exam?

  • Be vigilant with timing (you must attempt ALL questions)
  • Write legibly
  • Plan your answers
  • Structure sensibly
  • Work logically through all aspects of the question – be sure to ANSWER THE WHOLE QUESTION

*UPDATE* Health Knowledge have uploaded a page of useful information for Part A preparation here

Hot Topic: Advertising of Junk Food

Context for this post

As part of our Part A preparation, each week we write a briefing on a current ‘hot topic’ which has public health implications.  A useful document to guide choice of ‘hot topics’ has been the Faculty of Public Health  manifesto “12 Steps to Better Public Health”. Below is my briefing plus wider discussion on the advertising of junk food on TV.

FPH Manifesto Summary 

  • A recent Which? report criticised the 2006 Ofcom measures to ban junk food advertising between programmes where 20% of the audience were <16
  • The current measures are ineffectual and fail to cover programmes such as soaps which are still watched by large numbers of young people.
  • A complete ban before the 9pm watershed is needed to effectively reduce consumption of salt, saturated fats and sugars by children and adolescents, reducing the risk of cardiovascular disease later in life.

The problem

  • According to the Department of Health, £838million was spent on food and drink advertising in the UK in 2007.
  • Junk food marketing contradicts messages about healthy eating, undermining children’s ability to choose better food and parents’ efforts to feed them healthily.
  • To combat the rise of childhood obesity it is vitally important that children are persuaded to eat more healthily.
  • Food Standards Agency suggest junk food marketing directly influences children’s food preferences, and indirectly influences what family and friends consider to be a ‘normal’ diet. 

Current regulations

  • Prevent the advertising of “less healthy” products during children’s TV programming. 
    • 70 percent of the television that children watch is outside the hours of ‘children’s TV’ that these rules cover,
    • Research from Which? in 2007 reported that that 18 of the 20 most popular programs watched by children under 16 were not be covered.
  • Products must pass the FSA nutrient profile model prior to Ofcom giving permission to advertise.
    • Points are given for fat, salt and sugar.  If the score is too high, the product is not allowed to be advertised.

A solution being lobbied for is a 9pm watershed for junk food adverts, which has been suggested to have the following benefits:

  1. Protect children: This will eliminate over 80% of instances of kids watching junk food TV advertising.  The current rules only protect children from half this much advertising.
  2. Support parents: It will provide clarity on when junk food adverts will be shown, allowing parents to exercise responsibility over whether their kids see such adverts.  
  3. Improve children’s health: Estimates show that the health benefits from a 9pm watershed for junk food TV adverts will save the nation up to almost a billion pounds a year – at a cost to industry of  £130 million a year, and no cost to the government.

A series of surveys have demonstrated that the majority of parents are in favour of a protecting their children from junk food advertising:

  • BHF survey found that 68% of parents were in favour of pre-9pm junk food advertising restrictions, with only 7% against.
  • Which? found in 2006 that 79% of parents believe unhealthy foods should not be advertised during the times children are most likely to be watching television.

There are currently no legal restrictions on non-broadcast junk food marketing aimed at children. 

  • This category includes marketing through sponsorship, packaging, text messaging and the internet. 
  • This is a growing form of advertising aimed at children and its omission from statutory regulation is a loophole often exploited by food companies.

Therefore… there appears to be a lot of support for a 9pm watershed to be put into place, however what true effect this will have will be difficult to measure because there are so many sources of influence upon the perceptions and desires of children.  Although I feel supportive of this campaign, it’s underlying premise suggests that exposure to adverts is the most important factor… what other influences could be important?  Have these been considered?  E.g. previous exposure to different foods (e.g. junk vs. healthy), parenting styles, friends/peers at school, ease of accessing healthy food…etc.  In essense, my point is that this campaign will only be worthwhile if it is complimented by other relevant actions which encourage behaviour change as realistically we do not know which influences are the greatest (and influences are likely to be different for different individuals).  This policy is a step in the right direction, however when one considers that the sponsors for the 2012 Olympics include Coca-Cola, Cadburys and McDonalds (and no companies representing healthy consumption) one cannot help but be skeptical.


9/11 – Reflections from a Public Health Perspective

It’s September.  This is the month when the critical appraisal papers *generally* are chosen for the January Part A FPH Exam, *typically* from the BMJ… (so I’m making a mental-note to review papers which were published at the end of the month).  But this isn’t just any old September…

This September marks 10 years since the ‘9/11’ attacks.  On Tuesday September 11th 2001, four coordinated suicide attacks by al-Qaeda were carried out on the United States.  Reflections on this 10 year anniversary have started to creep onto all the news channels, and I assume these reflections will be sustained until the anniversary day (a week today). But how can a public health perspective contribute to such reflections…?

Handily, The Lancet have today published a series of articles which examine the health consequences of the 9/11 events.

The editorial “9/11 – Ten Years On” observed that the US governmental responce focused on defense, security and emergency preparedness.  Not only did this encourage fear and anxiety amongst the general population, it stigmatised many muslim communities and individuals.  Furthermore, health was pushed onto the backseat of political agendas, and the author suggests that:

“.. 9/11 was a huge opportunity cost for the health of the American people.”

As well as US domestic health effects, the 9/11 events have had international consequences.  One postitive outcome has been an increased post 9/11 commitment to global health encompassed by the US national security strategies.  The US recognised that investing in global health (amongst other aspects of development) had positive outcomes in terms of stability and security for their country. 

The 9/11 events, responses and consequences have provided a lot of reflections and learning for public health; from the micro (such as individual health protection, occupational health, suicide attacks), to the macro (health policies, emergency preparedness and reponse).

Articles of interest include:

“The events of 9/11 not only represent an example of a local act with global consequences, but also an instance where poverty and perceived injustice can contribute to catastrophic global instability and insecurity. It is now abundantly clear that human-made crises will, if not resolved decisively through politics and diplomacy, create the conditions for human-made disasters.”

And it begins…

Today we had the ‘Introduction to Part A’ day at the Deanery.  We had a brief overview from the TPD, a more detailed intro by a senior ST5 registrar and some advice from some ST3 survivers.

The day started postively, but we all left feeling absolutely knackered and a little overwhelmed.  To an extent it was information overload (in terms of the breadth of thngs we need to do) and the only ‘work’ was planning group study sessions in diaries, agendas and process things… but it’s kicking off so quickly.  Our first group session takes place next Tuesday 6th September and they’re on-going until 20th December…

For each session we will do 1) critical appraisal 2) review past papers relating to the ‘core topic’ that week 3) share notes on ‘hot topics’ 4) ‘statistic of the week’ and therefore have a large amount of ‘homework’ prep too.

…just as I was starting to enjoy having my weekends and evenings back… *sigh* …to the books!

Screening: Health Knowledge Interactive Learning Module

For those preparing for the FPH Part A exams, I strongly advise having a look around the Health Knowledge website.  It has pretty much *everything* we’ll need, including the ‘public health textbook’ (which correlates identically to the Part A syllabus) as well as a variety of self-taught modules.  There are plenty of further resources and it’s also user friendly.  Thumbs up from me!

Today, I thought I would re-familiarise myself with screening via the interactive module.

Chapters 1-5 cover the basic concepts, which is a bit simplistic if you’ve studied screening during a masters.  Chapters 6-8 I found interesting practical exercises and tips of how to apply knowledge and use public health skills (such as commissioning and working with media) and chapter 9 was an interview with Muir Grey where he described the changes and improvements made to screening programmes in the UK since the 70s.  Overall, quite a time-consuming unit!  A good overview, but feel I probably need to make notes explicitly related to the syllabus points to obtain the level of knowledge needed to survive a Part A question on screening.

What was refreshing however, was the practical element of this module.  Compared to the academic learning environment which prioritises test accuracy, today I learnt about the new (to me) concept of programme accuracy.  The main point to learn being a screening tool/test is only as good as the programme it is set in.  You may have a very accurate test, but if attention is not paid to training of staff who obtain the sample, communication between different agencies (such as labs, primary care…etc) is inadequate, and failsafe systems are not enforced, at best a screening programme is ineffective and at worst a serious incident (SI) can occur.