O que são experimentos de blocos aleatorizados e experimentos quadrado latino?

Alguns experimentos projetados podem efetivamente fornecer informações quando as medições forem difíceis ou caras ou podem minimizar o efeito da variabilidade indesejada na inferência do tratamento. O que se segue é uma breve discussão de dois experimentos comumente usados. Para mostrar estes experimentos, dois fatores de tratamento (A e B) e sua interação (A*B) são considerados. No entanto, estes experimentos não estão restritos a dois fatores. Se o experimento for balanceado, é possível usar ANOVA balanceada para analisar seus dados. Se não for, utilize GLM.

Experimento com blocos aleatorizados

A randomized block design is a commonly used design for minimizing the effect of variability when it is associated with discrete units (e.g. location, operator, plant, batch, time). The usual case is to randomize one replication of each treatment combination within each block. There is usually no intrinsic interest in the blocks and these are considered to be random factors. The usual assumption is that the block by treatment interaction is zero and this interaction becomes the error term for testing treatment effects. If you name the blocking variable as Block, the terms in the model would be Block, A, B, and A*B. You would also specify Block as a random factor.

Experimento Quadrado Latino com medidas repetidas

A repeated measures design is a design where repeated measurements are made on the same subject. There are a number of ways in which treatments can be assigned to subjects. With living subjects especially, systematic differences (because of learning, acclimation, resistance, and so on) between successive observations can be suspected. One common way to assign treatments to subjects is to use a Latin square design. An advantage of this design for a repeated measures experiment is that it ensures a balanced fraction of a complete factorial (that is, all treatment combinations represented) when subjects are limited and the sequence effect of treatment can be considered to be negligible.

Um experimento Quadrado Latino é um experimento com blocos com duas variáveis de bloco ortogonais. Em um experimento agrícola, pode haver gradientes perpendiculares que levem à escolha desses experimento. Para um experimento com medidas repetidas, uma variável de bloco é o grupo de indivíduos e a outra é o tempo. Se o fator de tratamento B possui três níveis, b1, b2 e b3, então uma das doze aleatorizações do quadrado latino possíveis dos níveis de B para grupos de sujeitos é:
Tempo 1 Tempo 2 Tempo 3
Grupo 1 b2 b3 b1
Grupo 2 b3 b1 b2
Grupo 3 b1 b2 b3

The subjects receive the treatment levels in the order specified across the row. In this example, group 1 subjects would receive the treatments levels in order b2, b3, b1. The interval between administering treatments should be chosen to minimize carryover effect of the previous treatment.

This design is commonly modified to provide information about one or more additional factors. If each group was assigned a different level of factor A, then information about the A and A*B effects could be made available with minimal effort if an assumption about the sequence effect given to the groups can be made. If the sequence effects are negligible compared to the effects of factor A, then the group effect could be attributed to factor A. If interactions with time are negligible, then partial information about the A * B interaction can be obtained. In the language of repeated measures designs, factor A is called a between-subjects factor and factor B a within-subjects factor.

It is not necessary to randomize a repeated measures experiments with a Latin square design.

Ao usar esse site, você concorda com a utilização de cookies para análises e conteúdo personalizado.  Leia nossa política