The term “monadic sequential” refers to how assessors evaluate – over time – two or more stimuli within the course of a study. In a monadic sequential presentation scheme, each assessor will be exposed to at least two objects but only one at a time, i.e., sequentially, and will provide their perceptions sequentially as well. For example, if two drinks, say ‘A’ and ‘B’, were to be evaluated by a sample of consumers, some would taste drink ‘A’ first, fill out the questionnaire, and then drink ‘A’ would be removed before they repeated the same steps with drink ‘B’. If the design was balanced for order of presentation, half of the consumers would taste ‘A’ first and half ‘B’ first.
In statistics, monadic sequential designs are also known as crossover designs.
Monadic sequential designs can be contrasted with Monadic studies in which each assessor only evaluates one stimulus. Home use tests (HUT) more often use a monadic design than do Central Location Tests (CLT). They also differ from so-called simultaneous presentation schemes, often employed in discrimination or paired preference tests, in which the panelist receives multiple samples at the same time. Of course, even in such cases, the assessor can only evaluate one sample at a time. The difference here is that all samples in the assessment set are evaluated prior to giving a response (say, a discrete choice) versus answering multiple questions about a given sample.
Monadic sequential only refers to the assessment process itself; it doesn’t imply anything specific about the type or nature of the sample design. For example, panelists may only evaluate a subset of all of the samples in the study, in which case the design is referred to as an Incomplete Block. Also, all possible presentation orders need not be present in the design. In fact, a consumer study with six samples would require 720 consumers in order to have every presentation order represented just once, which is far more consumers than is typically used in industry.
One variation of the monadic sequential approach is the Proto Monadic scheme in which, typically, the second sample is not evaluated using the same ballot that was used for the first sample, instead, a preference question is asked. The ballot portion would follow a typical monadic presentation scheme, whereas the preference question is similar to a standard comparative judgment in the sense that both samples are evaluated prior to making a choice.