A scale for assessing comfortability of car seats. The method description includes the steps to develop the scale, which are applicable for various other domains as well.
Development of seats in the automotive industry is expensive. Physical prototypes with adequate functionalities are required to carry out tests and evaluations, which are to a great extent devoted to improve the ergonomic features in order to reduce discomfort – and to improve comfort. However, the perceived differences of ergonomic features are often small and the results from comparisons of different seat concepts are rarely significant. The human body is very adaptive and not sensitive to distinguish variations in seats. The most important factor for assessing discomfort is time, which also contributes to costly evaluations. Today, the seats developed in the automotive industry fulfil basic ergonomic criteria for “good” sit comfort, which imply that seat design has little effect on the perceived discomfort, as long as there are no basic design flaws. Few studies have been able to produce significant differences in discomfort between different chair concepts (Helander and Zhang, 2001). From a cost-efficiency perspective it may not be justified to spend the resources on expensive seat variations of ergonomic features when the outcome is small and not significant.
The method for assessing seat comfort is a set of four steps:
1. Collect factors: Collect as many words as possible that can be related to comfort or discomfort through interviews with drivers, branch magazines, literature, dictionaries Google search etc. to find factors that cover as much of the meanings of comfort and discomfort as possible.
2. Reduce factors: Decide the criteria for reducing or replacing less useful factors, e.g. ambiguous factors, factors not applicable on new seats, such as dirty, worn out etc., extreme factors and useful in a seat evaluation, only one factor of the same word, no superlatives.
3. Separate factors: Carry out interviews with the relevant target group, e.g. truck drivers. Factors that are found to be related to, e.g. driver seats are divided into Comfort and Discomfort.
4. Group factors: Group the Comfort factors according to affinity and similarity. For this stage focus groups can be used to sort the factors into categories.
When the collection and compilation of the relevant factor are done, a sample of test persons evaluate different seat concepts and rate each factor on a seven point interval scale. The test persons can rate each factor with a stroke on the scale line for each seat concept, e.g. marking them with A and B respectively, which makes it easier for the test persons to do comparative ratings of the seat concepts.
The data is statistically treated using parametric tests (t-tests) and non-parametric tests (Wilcoxon signed rank test).
The underlying problem is that the terms “Comfort” and “Discomfort” are generally regarded as two end-points on the same scale. By defining “Comfort” and “Discomfort” as separate factors, where “Discomfort” refers to physical experiences and “Comfort” refers to mental impression of seats, much of the problems associated with sit comfort evaluation can be solved. Zhang et al. (1996) identified independent factors of comfort and discomfort in office chairs and found that the comfort factors were unaffected by the time spent in the chairs, implying that Comfort could be assessed immediately and, in addition, provide significant differences between chairs.
Moreover, two seats would elicit different ratings of Comfort, depending on, e.g. the aesthetics of the cloth material (Zhang et al, 1996). Thus, people’s impression of aesthetics affects their perception and preference of seats (Helander and Zhang, 2001).
For the automotive industry, this approach to Comfort and Discomfort give rise to more efficient and cost-efficient seat evaluations by focusing on the perceived comfort (unless there are no obvious violations of biomechanics design rules), with clear results in terms of the users’ preferences.
The collection of comfort factors and the process of reducing the collection of factors are critical in order to obtain factors that are relevant for the type of seat and for the sample of representative seat users. Moreover, it is important to secure that the evaluated seats fulfill the basic and the ergonomic/biomechanical criteria.
NB! It is important to keep in mind that this method does not replace research and evaluations for improved ergonomic/biomechanical features of seats.
References describing the method
Helander, M.G., Zhang, L. 2001. Forget about Ergonomics in Chair Design? Focus on Aesthetics and Comfort!, Proceedings of The International Conference on Affective Human Factors Design, Asean Academic Press, London.
Helander, M. G. and Zhang, L. 1997. Field studies of comfort and discomfort in sitting, Ergonomics, vol. 40, no. 9, pp. 895-915. Sohlman, H. and Staaf, H. (2006) Subjective Evaluations of Seat Comfort, identifying factors of comfort and discomfort in truck seats, Master thesis, Linköping university, Sweden