Experiments

Create an experiment description using CrossBlock or MultiCrossBlock, then generate trials for the experiment using synthesize_trials(). Print generated trials using print_experiments().

class sweetpea.Block

Abstract class representing an experiment description, which can be turned into a sequence of trials with synthesize_trials().

class sweetpea.CrossBlock(design, crossing, constraints, require_complete_crossing=True)

Creates an experiment description based on a set of factors and a subset them that are crossed.

The CrossBlock class is the main way of describing an experiment. The result is an object that be used with synthesize_trials() to generate trial sequences.

The design argument lists all of the factors in the experiment design. When a sequence of trials is generated for the experiment, each trial will have one level from each factor in design.

Different trial sequences generated from the experiment will have different combinations of levels in different orders. The factors in crossing supply an initial constraint, which is that every combination of levels in the crossing should appear once (within a sequence of trials that is determined the crossing size). When derived factors are included in a crossing, they effectively impose additional contraints, since each derived level is compatble with only certain levels of other factors. Finally, the constraints argument can impose additional constraints on the generated trials.

The number of trials in each generated trial sequence is also determined primarily by the crossing argument. Specifically, the number of trials starts as the product of the number of levels of the factors in crossing. This trial count can be adjusted by other elements of the design:

  • Exclude constraints in constraints can exclude levels of a crossed factor. In that case, as long as require_complete_crossing is set to false, combinations involving the factor are removed from the crossing.

  • When the levels of a derived factor in crossing have a window size N that is greater than 1, then N-1 additional preamble trials are normally added to the start of the sequence, so that the derived level is defined for the first trial that starts the crossing sequence. This behavior can be controlled in a derived factor by specifying a starting trial. When multiple derived factors are included in the crossing, the one with the latest starting trial determines the number of preamble trials. The levels of non-derived factors in the preamble trials are selected randomly and independently, except that the combinations are subject to any requirements from constraints, such as an AtMostKInARow constraint or an Exclude constraint.

  • When a derived-factor definition implicitly excludes certain combinations by the definition of its levels, then the number of trials in a crossing can be reduced, but only if require_complete_crossing is set to false.

  • A MinimumTrials in constraint can increase the number of trials in a sequence. Additional trials are added by scaling the weight of each crossing combination as many times as needed to meet or exceed the required number of trials. (To scale by repeating the crossing, instead, use Repeat.) Preamble trials are not scaled. If the minimum trial count minus preamble length is not a multiple of the crossing size, then it’s as if the minimum trial count were rounded up, and then trials are discarded at the end.

Parameters

  • design (List[Factor]) – the factors that make up the design

  • crossing (List[Factor]) – factors that are fully crossed in the block’s trials, which must be a subset of the design list

  • constraints (List[Constraint]) – constraints that every sequence of trials must satify; see Constraints

  • require_complete_crossing – dertermines whether every combination in crossing must appear in a block of trials; a false value is appropriate if combinations are excluded through an Exclude constraint

Returns

a block description

Return type

Block

class sweetpea.MultiCrossBlock(design, crossings, constraints, require_complete_crossing=True)

Creates an experiment description as a block of trials based on multiple crossings.

The MultiCrossBlock class generalizes CrossBlock, but it accepts multiple crossings in crossings, instead of a single crossing. Since MultiCrossBlock is more general, a CrossBlock instance is also a MultiCrossBlock instance.

The number of trials in each generated sequence for the experiment is determined by the maximum of number that would be determined by an individual crossing in crossings.

Every combination of levels in each individual crossing in crossings appears at least once within that crossing’s size. Smaller crossing sizes lead to replications of that crossing to meet the number of trials required for larger crossings. At the same time, different crossings in crossings can refer to the same factors, which creates constraints on how factor levels are chosen across crossings in a given trial.

Parameters

  • design (List[Factor]) – the factors that make up the design

  • crossings (List[List[Factor]]) – a list of crossings, where each crossing is a list of factors that are fully crossed in the block’s trials; the factors in each crossing must be a subset of the design list

  • constraints (List[Constraint]) – constraints that every sequence of trials must satify; see Constraints

  • require_complete_crossing – same as for MultiCrossBlock

Returns

a block description

Return type

Block

class sweetpea.Repeat(block, constraints)

Repeats the crossings of a given MultiCrossBlock (or CrossBlock) enough times to satisfy a minimum trial count specified in constraints. Unlike increasing the minimum trial count within block, levels are selected for each replication of the crossing independently, except that transition derived factors can create dependencies from one replication to the next.

Preamble trials are not replicated, since each replication of the crossing serves as a preamble for the next. If block contains multiple crossings, then all crossings must have the same preamble length.

Other constraints apply within each single repetetion or across the sequence of repetitions, depending on whether the constraint is specified as part block or provided in constraints for the repetition. For example, a Pin constraint within block applies to one trial of each repetition (where the index is relative to each repetition), while a Pin constraint in constraints applies to one trial for the entire trial sequence. When a crossing has preamble trials, constraints in block apply to a preamble for each repetition, which overlaps with the trials of the previous repetition. An Exclude constraint is not allowed in constraints, since that would imply changing the size of each repetition.

If constraints is empty, then the repetition has no effect, and generating trials from the repetition will be the same as generating them from block directly.

Parameters
Returns

a block description

Return type

Block

sweetpea.synthesize_trials(block, samples=10, sampling_strategy=IterateGen)

Given an experiment description, generates multiple blocks of trials.

Each block has a number of trials that is determined by the experiment’s crossing, and each trial is a combination of levels subject to implcit and explicit constraints in the experiment description.

The sampling_strategy argument determines properties of the resulting samples, such as whether each sequence reflects a uniformly random choice over all valid sequences. See Sampling Strategies for more information.

Note that the default sampling strategy does not provide a guarantee of uniform sampling. The default is chosen to produce a result as quickly as possible for the broadest range of designs.

Parameters

  • block (Block) – the experiment description

  • samples (int) – the number of sequences of trials to generate; for example, 1 sample would correspond to a single run of the experiment with a random ordering of the trials (subject to the experiment’s constraints)

  • sampling_strategy (Gen) – how a random set of trials is generated; the default is currently IterateGen, but this is subject to change

Returns

a list of blocks; each block is a dictionary mapping each factor name to a list of levels, where all of the lists in the dictionary have one item for each trial

Return type

List[Dict[str, list]]

sweetpea.print_experiments(block, experiments)

Prints the trials generated by synthesize_trials() in a human-readable format.

Parameters
sweetpea.tabulate_experiments(block=None, experiments, factors=None, trials=None)

Tabulates the number of times each crossing combination occurs in each sequence of experiments, and prints a summary in a human-readable format. This function might be used to check that synthesize_trials() produces an expected distirbution, for example.

Factors relevant to a crossing are normally extracted from block, but they can be specified separately as factors. When block is supplied, it must contain a single crossing, as opposed to a multi-crossing block produced by MultiCrossBlock.

Normally, all trails in each sequence are tabulate. If ‘trails` is provided, is lists trials that should be tabulated, and other trials are ignored. Trial indices in trials count from 0.

Parameters

  • block (Block) – the experiment description that was provided to synthesize_trials()

  • experiments (List[Dict[str, list]]) – sequences generated by synthesize_trials()

  • factors (List[Factor]) – an alernative to block supplying factors to use as a crossing

  • trials (List[int]) – the indices of trials to tabulate, defaults to all trials

sweetpea.save_experiments_csv(block, experiments, file_prefix)

Saves each sequence of experiments to a file whose name is file_prefix followed by an underscore, a number counting from 0, and “.csv”.

Parameters
sweetpea.experiments_to_dicts(block, experiments)

Converts a result from synthesize_trials(), where each generated sequence is represented as a dictionary of lists, so that each generated sequence is instead represented as a list of dictionaries.

Parameters
Returns

a list of lists of dictionaries, where each dictionary maps each factor name to the string name for the levels of the trial

Return type

List[List[Dict[str, Any]]]

sweetpea.experiments_to_tuples(block, experiments)

Converts a result from synthesize_trials(), where each generated sequence is represented as a dictionary of lists, so that each generated sequence is instead represented as a list of tuples.

Parameters
Returns

a list of lists of tuples, where each tuple contains the string names of levels selected for one trial

Return type

List[List[tuple]]