Chalder Fatigue Scale normative data is suspect
PACE set a threshold of 'normal' fatigue as a CFQ score of 18 or less, based on a mean of 14.2 and SD of 4.6, which were taken from a 2010 study,
Measuring fatigue in clinical and community settings. But these figures may well not be representative of the working age population.
The way they selected the participants is complex, and the underlying data was collected published in 1994:
Population based study of fatigue and psychological distress (T
Pawlikowska, T Chalder, S R Hirsch, P Wallace, D J M Wright, S C Wessely), and this is where the holes start appearing.
Crucial bit is point 4 if you're pressed for time.
1. This does not appear to be a representative sample
They mailed registered patients at several different types of practice, but made no attempt to match them to the population (vs Bowling who showed SF36 data was based on a cohort well-matched with census data).
We sent questionnaires to all patients aged 18 to 45 years registered with selected practices, 3 from London and three from rural or semirural settings.
Two practices (subsequently referred to as practice 1) were working from the same health centre in south London in a mixed urban community with a large amount of temporary accommodation. Practice 2 was an inner city London practice where most patients were socially deprived. The third practice was located on the Surrey-Hampshire border with patients predominantly in socioeconomic classes II and III. Practice 4 was in an urban area of a south coast port, and the last practice was in a Somerset village, with a static close knit community and many stable families. The total number of patients aged 18-45 registered with the practices was 31 651 (15 222 men, 16 429 women).
- They restriced it to patients age 18-45, though didn't explain why (though this particular feature is likely to bias towards healthier individuals).
2. Low response rates could lead to biased findings
The response rate was only 48%. After investigating non-responders they found that many had moved (a known problem with GP practices, esp in urban areas) and estimated the response rate from those who received the questionnaire was 64%. The issue is, were people who were less well, or fatigued, more likely to respond to questionnaires about fatigue and health?
By comparison, the Bowling SF36 data used face to face interviews, with a 78% response rate and the SF36 qs were part of a much broader questionnaire including lifestyle and finance - so healthy people are less likely to ignore because it doesn't apply to them. The Jenkinson SF36 figures had a 72% response rate, but this is of those mailed. Let's say 5% of the orginal list they mailed was incorrect/moved (quite a cautious assumption from my direct marketing experience) giving a net response rate of 76% - and again this was part of a larger survey including lifestyle, reducing the chance of healthy people opting out.
ETA: however,
this study suggests that ill people might not be more likely to respond, though it does relate to questions on "subjective well-being (overall life satisfaction and self-assessed health)" rather than just health.
3. Only participants who visited their GP were included
To complicate things, Cella didn't use all the data from the original mailing. Instead, data was only used from respondents who subsequently visited their GP about a viral or other complaint
and were selected as part of another study. So anyone who was very healthy and never visited their GP would not be included. Those who visited their GP more often would consequently have more chances to make this cohort than those who rarely visited their GP. All of this is likely to bias the sample against healthy individuals.
Precise figures are not given for the original 1994 study but from the figures they give it looks like the mean is very close to 13.6, compared to the 14.2 quoted by Cella for his sub-group, suggesting at least some bias here.
ETA I've found the fatigue case data for the Cella study (
Postinfectious fatigue: prospective cohort study in primary care, p1335 under "stage 2 sample"): it gives 42.6% caseness, vs 38% for Pawlikowska, confirming the Cella cohort is more fatigued than the Pawlikowska one.
4.Data from the original study indicate this is an unhealthy cohort.
According to Pawlikowska, 38% of patients had a score about the original Chalder bimodal cut off of 3 (as used in the PACE protocol) and
18.3% of patients were substantially fatigues for 6 months or longer. Whoa, that looks unhealthy, esp as the paper quotes a
1990 paper that found
only 10% of GP practice patients had fatigue for one month or more. I think there are some US studies indicating fatigue of over 6 months in the population is much less than 18%.
So, I'm pretty fatigued now, and so are you if you've read this far. But it looks like PACE have been using highly unsuitable 'normative' data. Again.
ETA: should mention that the Cella CFQ data is not normally distributed and therefore, like the SF36 data, is not suitable for use with parametic stats, such as the 'within 1 SD of the mean' formula used by PACE:
Similarly to the CFS group, the community group scores were not distributed normally but were positively skewed. Values of skewness for the nonclinical group ranged between 0.40 (Item 5) and 1.06 (Item 9) with a mean skewness of 0.77.
