Comparison of two different approaches for the analysis of data from a prospective cohort study: an application to work related risk factors for low back pain

W E Hoogendoorn; P M Bongers; H C W de Vet; J W R Twisk; W van Mechelen; L M Bouter

doi:10.1136/oem.59.7.459

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Comparison of two different approaches for the analysis of data from a prospective cohort study: an application to work related risk factors for low back pain

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Generalised estimating equations and low back pain: a response to Hoogendoorn et al.
Elaine F Harkness
Published on: 6 November 2002
TABLE 1 (Part i): Harkness et al response to Hoogendoorn et al.
Elaine F Harkness
Published on: 6 November 2002
TABLE 1 (Part ii) Harkness et al. response to Hoogendoorn et al.
Elaine F Harkness
Published on: 6 November 2002
TABLE 2: Harkness et al. response to Hoogendoorn et al.
Elaine F Harkness
Published on: 6 November 2002

Published on: 6 November 2002
Generalised estimating equations and low back pain: a response to Hoogendoorn et al.
- Elaine F Harkness, PhD Student, ARC Epidemiology Unit
- Other Contributors:
  Elizabeth S Nahit, Gary J Macfarlane, Alan J Silman, Graham Dunn, John McBeth
Dear Editor
We read with interest the article by Hoogendoorn et al. (2002) who examined the use of different approaches to analysing data from their prospective cohort study of work-related exposures and the future onset of low back pain.[1]
Exposures and outcomes are time dependent factors as they are subject to change over time. The strength of the relationship depends on the assumptions of time depend...
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Dear Editor
We read with interest the article by Hoogendoorn et al. (2002) who examined the use of different approaches to analysing data from their prospective cohort study of work-related exposures and the future onset of low back pain.[1]
Exposures and outcomes are time dependent factors as they are subject to change over time. The strength of the relationship depends on the assumptions of time dependence (or independence) of exposures and outcomes. The effects of these assumptions can be investigated by adopting different modelling approaches to studies that have collected repeated measures of exposure and outcome data over time.
Hoogendoorn et al. (2002) have adopted such an approach in their study of work-related risk factors for low back pain. Information on work-related physical and psychosocial factors and low back pain outcome was collected at baseline and in three annual follow-ups. They demonstrated an increased risk of low back pain for work-related mechanical factors, when using two different generalised estimating equation (GEE) models compared to the standard logistic regression approach.[1] Conversely, for work-related psychosocial factors the association with low back pain was weaker when the GEE method was employed. Such an approach is enlightening and we agree that it is important to explore such analytical techniques in the investigation of work-related risk factors and musculoskeletal symptoms. Therefore further exploitation of this method of analysis seems appropriate.
We have recently conducted a prospective study of new onset low back pain in 1081 newly employed workers from twelve occupational settings.[2] We examined newly employed workers since studies conducted in well established work forces may be influenced by the healthy worker effect, whereby workers may have changed their job or certain aspects of their job as a result of musculoskeletal pain. In brief, at baseline subjects completed a questionnaire, including an assessment of pain status. A pre-shaded manikin was used to enquire about low back pain, defined as pain between the 12th rib and the gluteal folds, lasting at least 24 hours in the past month. Individuals free from low back pain at baseline were identified and followed up at 12 and 24 months. The detailed questionnaire also gathered information on a number of work-related mechanical and psychosocial exposures.
The models used for analysis were identical to those used by Hoogendoorn et al. (2002). The standard logistic regression model was used to examine the relationship between exposures and new onset low back pain at 12 or 24 months, in those free from low back pain at baseline. GEE models are used to analyse repeated measures data, by taking the within subject correlation into account, and providing a summary estimate over time. In GEE Model 1, the relationship between baseline exposures and new onset low back pain at 12 and 24 months was examined. In GEE Model 2 that relationship was examined for baseline exposures and new onset low back pain at 12 months, and 12 month exposures and new onset low back pain at 24 months.
The two models in which risk factors were assumed to be time independent (standard logistic regression and GEE Model 1) produced similar point estimates for developing new onset low back (Table 1 Part i, Part ii and Table 2), with narrower 95% confidence intervals for GEE. Model 1. In GEE Model 2, where risk factors are assumed to be time dependent, differences were noted for only a small number of variables (carrying on one shoulder, lifting at or above shoulder level, general health questionnaire). In addition, the 95% confidence intervals were narrower than those from the standard logistic regression and GEE Model 1. However, there was no consistent pattern of attenuation or growth noted in either the mechanical or psychosocial risk factors examined. (Table 1 Part i, Part ii and Table 2).
In summary, we agree that it is important to investigate different statistical techniques in an attempt to determine what effect the assumptions of time dependence (or independence) have on predictors of musculoskeletal pain. However, unlike the study by Hoogendoorn et al. (2002), our data show that the choice of model has relatively little influence on the magnitude of the results, although GEEs give more accurate estimates.
References
(1) Hoogendoorn WE, Bongers PM, de Vet HC, Twisk JW, van Mechelen W, Bouter LM. Comparison of two different approaches for the analysis of data from a prospective cohort study: an application to work related risk factors for low back pain. Occup Environ Med 2002;59:459-465.
(2) Nahit ES, Macfarlane GJ, Pritchard CM, Cherry NM, Silman AJ. Short term influence of mechanical factors on regional musculoskeletal pain: a study of new workers from 12 occupational groups. Occup Environ Med 2001;58:374-381.
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Conflict of Interest:
None declared.
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Published on: 6 November 2002

TABLE 1 (Part i): Harkness et al response to Hoogendoorn et al.

Elaine F Harkness, PhD Student, ARC Epidemiology Unit
Other Contributors:
- Elizabeth S Nahit, Gary J Macfarlane, Alan J Silman, Graham Dunn, John McBeth

Table 1(i): Work-related mechanical risk factors and new onset low back pain*
...

Table 1(i): Work-related mechanical risk factors and new onset low back pain*
Univariate associations
	Logistic regression		GEE Model 1		GEE Model 2
Exposure	OR	95% CI	OR	95% CI	OR	95% CI
Manual handling activities
Lifting with one hand
Never	1	Referent	1	Referent	1	Referent
≤ 15kg	2.01	1.26-3.21	1.74	1.15-2.64	1.37	0.91-2.07
> 15kg	1.92	1.05-3.51	1.75	1.01-3.03	1.62	0.98-2.67
Lifting with two hands
Never	1	Referent	1	Referent	1	Referent
≤ 24kg	1.68	1.04-2.71	1.63	1.06-2.51	1.34	0.87-2.06
> 24kg	2.50	1.43-4.37	2.27	1.38-3.72	1.78	1.11-2.85
Carrying on one shoulder
Never	1	Referent	1	Referent	1	Referent
≤ 30 kg	0.81	0.42-1.56	0.81	0.44-1.48	1.12	0.65-1.93
> 30 kg	1.46	0.71-3.04	1.39	0.71-2.75	1.28	0.67-2.44
Lifting at or above shoulder level
Never	1	Referent	1	Referent	1	Referent
≤ 23 kg	1.81	1.02-3.21	1.75	1.05-2.93	1.56	0.93-2.63
>23 kg	3.48	1.71-7.08	3.17	1.66-6.06	2.12	1.18-3.81
Pushing
Never	1	Referent	1	Referent	1	Referent
≤ 65kg	1.63	0.94-2.81	1.52	0.93-2.49	1.34	0.83-2.14
> 65kg	1.69	0.95-3.01	1.51	0.90-2.54	1.32	0.78-2.22
Pulling
Never	1	Referent	1	Referent	1	Referent
≤ 56kg	2.01	1.06-3.79	1.95	1.10-3.46	1.53	0.90-2.57
> 56kg	2.96	1.68-5.23	2.72	1.63-4.52	2.05	1.23-3.42
* Adjusted for gender, age group and occupation

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Conflict of Interest:
None declared.

Published on: 6 November 2002

TABLE 1 (Part ii) Harkness et al. response to Hoogendoorn et al.

Elaine F Harkness, PhD Student, ARC Epidemiology Unit
Other Contributors:
- Elizabeth S Nahit, Gary J Macfarlane, Alan J Silman, Graham Dunn, John McBeth

Table 1(ii): Work-related mechanical risk factors and new onset low back pain*
Univariate associations

...

Table 1(ii): Work-related mechanical risk factors and new onset low back pain*
Univariate associations

	Logistic regression			GEE Model 1		GEE Model 2
Exposure	OR		95% CI	OR	95% CI	OR	95% CI
*Posture*
Sitting
Do not sit as part of job	1		Referent	1	Referent	1	Referent
< 2 hours	0.57		0.33-1.00	0.59	0.35-0.98	0.99	0.61-1.61
≥ 2 hours	0.62		0.37-1.02	0.64	0.41-1.01	0.89	0.55-1.42
Standing
Do not stand as part of job	1	Referent		1	Referent	1	Referent
<15 minutes	1.42	0.67-3.03		1.42	0.71-2.83	1.09	0.57-2.06
15 minutes - < 2 hours	1.98	0.95-4.10		2.00	1.03-3.89	1.55	0.83-2.88
≥ 2 hours	2.37	1.09-5.13		2.27	1.12-4.59	1.75	0.90-3.40
Drive as part of job
no	1	Referent		1	Referent	1	Referent
yes	1.29	0.66-2.51		1.27	0.69-2.33	1.14	0.67-1.94
Kneeling
Never	1	Referent		1	Referent	1	Referent
<15 mins	1.02	0.61-1.70		1.00	0.63-1.60	1.40	0.91-2.16
≥15 mins	1.85	1.04-3.26		1.79	1.07-2.97	2.05	1.28-3.28
Squatting
Never	1	Referent		1	Referent	1	Referent
<15 mins	1.40	0.86-2.27		1.37	0.88-2.11	1.11	0.73-1.70
≥15 mins	2.19	1.19-4.04		2.12	1.22-3.67	1.81	1.08-3.05
Bending
Never	1	Referent		1	Referent	1	Referent
< 15 mins	0.95	0.60-1.51		1.05	0.70-1.59	1.57	1.06-2.31
≥15 mins	1.27	0.80-2.01		1.31	0.87-1.97	1.26	0.83-1.93
Stretching below knee level
Never	1	Referent		1	Referent	1	Referent
<15 mins	1.42	0.93-2.17		1.43	0.97-2.10	1.34	0.92-1.94
≥15 mins	1.28	0.67-2.45		1.22	0.67-2.22	0.91	0.50-1.68
Working with hands above shoulder
Never	1	Referent		1	Referent	1	Referent
<15 mins	1.40	0.86-2.27		1.40	0.90-2.18	1.60	1.06-2.42
≥15 mins	1.49	0.91-2.45		1.43	0.91-2.24	1.56	0.99-2.46
Repetitive arm/wrist movements
Never	1	Referent		1	Referent	1	Referent
<2 hours	1.58	0.96-2.62		1.52	0.96-2.40	1.40	0.90-2.17
≥ 2 hours	1.45	0.86-2.45		1.36	0.85-2.20	1.27	0.80-2.01

* Adjusted for gender, age group and occupation

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Conflict of Interest:
None declared.

Published on: 6 November 2002

TABLE 2: Harkness et al. response to Hoogendoorn et al.

Elaine F Harkness, PhD Student, ARC Epidemiology Unit
Other Contributors:
- Elizabeth S Nahit, Gary J Macfarlane, Alan J Silman, Graham Dunn, John McBeth

Table 2: Work-related psychosocial risk factors and new onset low back pain*
...

Table 2: Work-related psychosocial risk factors and new onset low back pain*
Univariate associations
	Logistic regression		GEE Model 1		GEE Model 2
Exposure	OR	95% CI	OR	95% CI	OR	95% CI
*Job Demand*
Stressful work
Never / occasionally	1	Referent	1	Referent	1	Referent
At least half of the time	1.74	1.08-2.80	1.63	1.07-2.47	1.59	1.06-2.37
Monotonous work
Never / occasionally	1	Referent	1	Referent	1	Referent
At least half of the time	1.88	1.07-3.30	1.87	1.13-3.08	1.94	1.22-3.09
Hectic work
Never / occasionally	1	Referent	1	Referent	1	Referent
At least half of the time	1.79	1.20-2.67	1.61	1.13-2.29	1.23	0.86-1.78
*Job satisfaction*
Job satisfaction
Not dissatisfied	1	Referent	1	Referent	1	Referent
(Very)/dissatisfied	0.97	0.28-3.32	0.98	0.32-3.03	1.34	0.60-2.99
*Social support*
Support from colleagues
Not dissatisfied	1	Referent	1	Referent	1	Referent
(Very)/dissatisfied	2.40	0.62-9.32	2.00	0.66-6.06	1.85	0.73-4.70
*Control over work*
Control over own work
At least sometimes	1	Referent	1	Referent	1	Referent
(Very)/seldom	0.95	0.51-1.79	0.90	0.50-1.61	0.74	0.37-1.45
Learn new things
At least sometimes	1	Referent	1	Referent	1	Referent
(Very)/seldom	1.84	0.72-4.70	1.63	0.70-3.81	1.65	0.77-3.52
*Individual distress (GHQ)*
GHQ total
0	1	Referent	1	Referent	1	Referent
1-2	1.08	0.70-1.68	1.09	0.73-1.62	0.97	0.65-1.46
≥3	2.51	1.52-4.13	2.26	1.47-3.48	1.44	0.94-2.20
* Adjusted for gender, age group and occupation

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Conflict of Interest:
None declared.

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