P. L. C. Van Geert

Focus on variability: New tools to study intra-individual variability in developmental data

Abstract In accordance with dynamic systems theory, we assume that variability is an important developmental phenomenon. However, the standard methodological toolkit of the developmental psychologist offers few instruments for the study of variability. In this article we will present several new methods that are especially useful for visualizing and describing intra-individual variability in individual time-serial data of repeated observations. In order to illustrate these methods, we apply them to data of early language development. After reviewing the common techniques and measures, we present new methods that show variability in developmental time-series data: the moving min–max graph, and the progmax–regmin graph. In addition, we demonstrate a technique that is able to detect sudden increases of variability: the critical frequency method. Also, we propose a technique that is based on a central assumption of the measurement-error-hypothesis: namely the symmetric distribution of error. Finally, as traditional statistical techniques have little to offer in testing variability hypotheses, we examine the possibilities that are provided by random sampling techniques. Our aim with the present discussion of variability and the demonstration of some simple yet illustrative techniques is to help researchers focus on rich additional sources of information that will lead to more interesting hypotheses and more powerful testing procedures, adapted to the unique nature of developmental data.

A longitudinal study of basal cortisol in infants : Intra-individual variability, circadian rhythm and developmental trends

Abstract Mothers with normally developing babies were visited in their homes during 13 consecutive weeks, when the babies were around 5–8 months of age. Basal salival cortisol measures were taken for both the baby and the mother on arrival. The infants’ basal cortisol decreased linearly with age, was negatively related to sleep, and did not show adult-like circadian declines from morning to mid-afternoon. Furthermore, while the infants showed relative stability across individuals, they displayed great intra-individual variability across assessments. Contrarily, the mothers displayed important inter-individual variability, together with a relative stability across assessments. The infants’ important intra-individual variability was not affected by gender, nor time of visit, nor was it related to the mothers’ basal cortisol. Daily measures of basal cortisol taken in a subgroup of infants indicated the day-to-day intra-individual variability to be of the same magnitude as the week-to-week variability. The question of how the intra-individual variability in basal cortisol affects assessments of cortisol in infancy is addressed. The aggregation of data with the goal of increasing the reliability of the assessments is shown to be inadequate for infant basal cortisol.

A dynamic developmental model of suicide - Commentary

Based on the work by Prigogine and colleagues from physics and other ‘hard’ sciences, a method is presented for studying and developing models of the dynamics of developmental changes in complex open

The Concept of Development and the Structure of Developmental Theories

Abstract Theories of development often use very different meanings of the word “development” (of. Overton, 1984. In this paper, I shall present ways to analyze some of the conceptual differences covered by the term “development,” and by related terms such as “learning.” I shall also give an example of how the structure of a developmental theory can be explained starting from the theorys definition of development (a more elaborate statement of the present ideas can be found in van Geert, 1986a, 1986b).

Weg met de psychologie – voor een honderdste seconde, zo ongeveer…

Het artikel ‘Weg met de psychologie!’ (Lamme, dit tijdschrift) laat mij achter met een zeer ambivalent gevoel. Het is een leuk stuk om te lezen, bevat een aantal heel interessante wetenswaardigheden over visuele waarneming, geheugen en bewustzijn en een aantal algemene uitspraken over de psychologie. En nu hoop ik van harte dat ik niet begrijp waar het bij die laatste om gaat, dat ik iets cruciaals in de redenering van Lamme heb gemist. Ik hoop dat wat in de tekst herhaaldelijk ‘de psychologie’ wordt genoemd verwijst naar een aantal theorieën of opvattingen die binnen het vakgebied van de orator geponeerd worden en die door zijn onderzoek, waar ik alle respect voor heb, onderuit zijn gehaald. Als dat zo is, dan is de titel van de oratie eigenlijk een iets te elliptische, ironische manier om te zeggen: ‘Weg met een aantal psychologische opvattingen die binnen mijn vakgebied van de cognitieve neurowetenschap niet houdbaar zijn gebleken.