BackgroundIn this paper, the basic elements related to the selection of participants for a health research are discussed. Sample representativeness, sample frame, types of sampling, as well as the impact that non-respondents may have on results of a study are described. The whole discussion is supported by practical examples to facilitate the reader's understanding.ObjectiveTo introduce readers to issues related to sampling.

BackgroundThe purpose of this paper was to review the literature of the cohort studies which evaluated the association between physical activity during the life course and bone mineral content or density in young adults.MethodsProspective cohort studies with bone mineral density or content measured in the whole body, lumbar spine and femoral neck by dual energy x-ray absorptiometry as outcome and physical activity as exposure were searched. Two independent reviewers selected studies retrieved from electronic databases (Medline, Lilacs, Web of Science and Scielo) and reviewed references of all selected full text articles. Downs & Black criterion was used in the quality assessment of these studies.ResultsNineteen manuscripts met inclusion criteria. Lumbar spine was the skeletal site most studied (n = 15). Different questionnaires were used for physical activity evaluation. Peak strain score was also used to evaluate physical activity in 5 manuscripts. Lack of statistical power calculation was the main problem found in the quality assessment. Positive associations between physical activity and bone mass were found more in males than in females; in weight bearing anatomical sites (lumbar spine and femoral neck) than in total body and when physical activity measurements were done from adolescence to adulthood – than when evaluated in only one period. Physical activity during growth period was associated with greater bone mass in males. It was not possible to conduct pooled analyses due to the heterogeneity of the studies, considering mainly the different instruments used for physical activity measurements.ConclusionsPhysical activity seems to be important for bone mass in all periods of life, but especially the growth period should be taking into account due to its important direct effect on bone mass and its influence in physical activity practice in later life. Low participation in peak strain activities may also explain the lower number of associations found in females.

Resumo O objetivo deste estudo foi avaliar a adequação do cuidado pré-natal no Brasil associado a determinantes sociodemográficos. A pesquisa consistiu em uma análise dos dados da Pesquisa Nacional de Saúde realizada no Brasil em 2013. Foram avaliados dois desfechos sobre a adequação do pré-natal, o índice de Kessner modificado por Takeda que, além de levar em consideração esse índice, avaliou se houve aferição da pressão arterial e do peso em todas as consultas, realização de algum exame de sangue e urina e ultrassom. Ambos indicadores de qualidade foram avaliados para o Brasil e também pelas macrorregiões do país. De acordo com o desfecho 1, 80,6% das mulheres realizaram o pré-natal adequado. Ao adicionarmos a realização de exames (Desfecho 2) o percentual foi 71,4%. O pré-natal adequado foi mais frequente entre as mulheres de cor branca e que realizaram o pré-natal na rede privada. A região norte apresentou as menores frequências de pré-natal adequado, enquanto a região sudeste as maiores. Apesar da ampla cobertura, o pré-natal no Brasil ainda apresenta iniquidades e baixa qualidade no atendimento, especialmente entre mulheres das regiões mais pobre do país.

The importance of estimating sample sizes is rarely understood by researchers, when planning a study. This paper aims to highlight the centrality of sample size estimations in health research. Examples that help in understanding the basic concepts involved in their calculation are presented. The scenarios covered are based more on the epidemiological reasoning and less on mathematical formulae. Proper calculation of the number of participants in a study diminishes the likelihood of errors, which are often associated with adverse consequences in terms of economic, ethical and health aspects.