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The objective of this article is to analyze the quality of the information collected by a self-administered survey addressed to citizens of Andalusia, who were offered the possibility of answering using the post or Internet. The study showed the advantages of using web-based self-administered questionnaires. Web surveys showed a low number of unanswered questions, more detailed answers to open questions, and longer answers to questions than those generated from paper questionnaires. The five open questions with text showed longer answers from the web survey, around 63 characters more. In the open questions with numerical answers, the use of drop-box (select list) generated better response than the use of a blank space left in the paper questionnaire.

The aim of this paper is to compare sample quality across two probability samples and one that uses probabilistic cluster sampling combined with random route and quota sampling within the selected clusters in order to define the ultimate survey units. All of them use the face-to-face interview as the survey procedure. The hypothesis to be tested is that it is possible to achieve the same degree of representativeness using a combination of random route sampling and quota sampling (with substitution) as it can be achieved by means of household sampling (without substitution) based on the municipal register of inhabitants. We have found such marked differences in the age and gender distribution of the probability sampling, where the deviations exceed 6%. A different picture emerges when it comes to comparing the employment variables, where the quota sampling overestimates the economic activity rate (2.5%) and the unemployment rate (8%) and underestimates the employment rate (3.46%).

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ResumenEn el presente trabajo se exponen diversas situaciones en las que no se respeta una de las especificaciones básicas del muestreo probabilístico, la selección basada en la equiprobabilidad, centrando nuestra atención en cómo este hecho influye en la capacidad de inferencia estadística de los resultados obtenidos en la investigación social mediante encuesta. Tras la exposición de los tipos de errores susceptibles de cometerse en la investigación mediante encuesta, centraremos nuestro análisis en la «no respuesta total», un fenómeno que altera la probabilidad de selección de los entrevistados y con una importancia creciente en los últimos años. El trabajo termina presentando un modelo de reducción de la no respuesta en la encuesta postal. Palabras clave: calidad en la encuesta, error de medida, error de cobertura, error muestral, error de no respuesta. Abstract. Nonsampling errors and survey qualityThis article provides a review of recent research on the quality of survey data. The paper reviews standard methodological treatments of survey errors and discusses the importance of developing population models for measurement errors. A model is presented which nests measurement errors within other non measurement survey errors: errors of coverage, sampling, and non response. The third and following section of the article focus the attention on non response.