Physical activity in young people has become a major issue in public health as evidence emerges on the important role of physical activity in many health conditions, including overweight and obesity, type 2 diabetes, cardiovascular disease risk, skeletal health, and mental health. In particular, the issue of obesity in youth, and the link between this condition and type 2 diabetes, as well as the increases in diabetes  is topical and currently demanding much attention in physical activity research .
Establishing links between physical activity and health outcomes is a fundamental phase of the behavioural epidemiology framework proposed by Sallis and Owen . An early phase of this framework is to identify valid and reliable ways to assess physical activity. If suitable assessment methods can be developed, and health outcomes identified that are associated with physical activity, this logically leads to research identifying factors associated with physical activity ('correlates') and interventions to increase physical activity. However, much of this is predicated on the use of suitable tools for assessing levels of physical activity.
In addition to assessing physical activity in research studies, where researchers may have the opportunity of conducting lengthy and detailed assessments, tools are also needed for assessing population level prevalence . This is the focus of the present paper. Usually such instruments have to be brief as they may sit alongside other health assessments within population surveillance systems .
Assessment Using Self-Report
Until the development of movement sensors, such as pedometers and accelerometers, the assessment method of choice for physical activity has been self-report. Consequently, there are a large number of instruments, with varying degrees of formatting and development, with many aimed at assessment in young people. However, this approach to measurement is fraught with difficulties and consequently there are many poorly developed instruments alongside those that have a history of more robust design and development.
There are many good sources of discussion concerning issues in self-reported assessment of physical activity, in both youth and adults (e.g., [6–10]). Moreover, there are many methods that can be used, including questionnaires/surveys and diaries. Survey methods can require self or proxy completion, the latter, for example, by parents for young children.
In the analysis of self report surveys that follows, we focused on instruments that might be suitable for assessment in large-scale population surveillance, usually for the purpose of estimating the prevalence of physical activity. Prevalence estimates refer to the proportion of the population estimated within physical activity time-based categories (e.g., percentage meeting national guidelines, such as 60 minutes of MVPA each day of the week). This contrasts with possible outputs such as mean estimates of time spent in specific types of physical activity behaviours, or total physical activity behaviour, or 'dose'.
Such assessments are usually part of a wider public health surveillance system. The USA Centres for Disease Control  stated that "Public health surveillance systems should be evaluated periodically, and the evaluation should include recommendations for improving quality, efficiency, and usefulness." This will include an evaluation of the type of assessment (e.g., objective or self-report), and continued assessment of the nature of the instrument being used. To this end, Project ALPHA was established, with one aim being to provide guidance on the measurement of physical activity in young people using self-report instruments and assess their suitability for population surveillance monitoring, particularly in the European context (see http://sites.google.com/site/alphaprojectphysicalactivity/). That is, instruments relevant for populations of these types of countries.
In evaluating, as well as developing, self-report instruments for surveillance of physical activity in youth, consideration needs to be given to several key issues , including:
What domains of physical activity are being assessed? These could be general (total physical activity) or in specific contexts, such as school or leisure-time.
Does the instrument assess the frequency, intensity, duration and type of activity?
Does the instrument assess the temporal dimension of physical activity (e.g., activity at different times of the day)?
Over what period are participants being asked to recall their activity?
Is the instrument suitable for the age group it is aimed at? This will require the testing of items for appropriateness of the language used and consideration of the cognitive capacities of the child being assessed.
Is the instrument appropriate in respect of ease of completion and participant burden, given that large samples will be required to be tested for population surveillance and prevalence?
Is the instrument suitably valid and reliable?
This paper addresses the assessment of physical activity for children and adolescents for population level surveillance using self-report instruments. With technological improvements and reductions in costs, population surveillance may, in future, routinely use more objective instruments, such as accelerometers. However, even if this is the case, concomitant assessment of some aspects of physical activity (e.g., type) will require self-report assessment, alongside objective methods.
The purpose of this paper, therefore, is to review existing self-report instruments purporting to assess physical activity in young people. Instruments will be appraised and a short list of measures that may be suitable for population surveillance, with the ability to provide suitable prevalence estimates, will be considered with the purpose of making recommendations on usage, particularly in a European context.