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  • Glossary

Seriate

To perform a seriation, choose Seriate from the Seriate menu. This will bring up the OptiPath Seriation dialog. Some summary information about your data is presented. This includes the numbers of artifacts (or assemblages) and features in your data. You are also shown the earliest and latest allowed dates for this seriation. Clicking the Compute button will start the seriation. For information on objectives and techniques for to create a good seriation, see Seriating.

Seriate Menu

Seriate initiates the calculation of a new seriation. There is a corresponding Seriate button on the toolbar.

Reverse reverses the order of the current seriation. Seriation is an ordering technique, not a dating technique; and the direction of the ordering is arbitrary without additional information. Shortest Path seriation is able to assign relative dates to the items being seriated; however, this depends upon the dates you enter as the Earliest and Latest dates for the seriation. Even then, without Earliest and Latest dates for individual artifacts, OptiPath is not able to orient the direction of the relative dating - i.e. OptiPath does not know which end is earliest and which is latest. The Reverse command allows you to reverse the order of the current seriation, as seen in the Results table. There is a corresponding Reverse button on the toolbar.

Objective allows the user to choose the objective to be used in determing a best possible seriation. See Objectives for more information.

Break Ties instructs OptiPath to use a secondary objective fuction to break ties. If the user is using Unimodality as the Objective, ties will be broken using the Path Length. If the Objective is Path Length, or Average, Maximum or Squared Rates of Change, then Unimodality is used to break ties.

Use Dates tells OptiPath to use the user entered Earliest and Latest dates for each artifact (assemblage). This option has two purposes. First, not using input dates allows you to see how OptiPath would seriate your items without these constraints without having to go through all the items and delete their Earliest and Latest dates (and re-enter them later when you want to use them again). Second, OptiPath in this case will report Date Errors in the Results table and the Artifacts) Assemblages) table.

Help will bring up this documentation.

Seriate Dialog

Features displays the number of features being considered in the seriation.

Artifacts/Assemblages displays the number of artifacts (assemblages) being considered in the seriation.

Earliest Date is the earliest date that will be assigned to an artifact in the seriation. This is for your information only, the Earliest date can be modified in the Seriations table.

Latest Date is the latest date that will be assigned to an artifact in the seriation. This is for your information only, the Latest date can be modified in the Seriations table.

Start is the length of the path determined by the starting seriation.

Current is the length of the path for the current seriation.

Improvement is the improvement in the path length between the current and the starting seriation.

Effort is the amount of effort you want the algorithms to devote in looking for an optimal answer (determined by the objective). The seriation problem is modeled as a special case of the "traveling salesman problem", which is notoriously difficult to solve. A seriation of a few dozen artifacts could take an inordinate amount of time (hours or more) to solve optimally on a desktop computer. OptiPath uses an heuristic optimization algorithm that can run quickly although it is not guaranteed to find the very best possible answer. For large samples, even a heuristic can require substantial amounts of computation time. The Effort parameter allows you to control the amount of computation time: the smaller the value of Effort, the quicker the computation and possibly the less optimal the answer. A computation can be suspended or abandoned in mid-stream without losing the benefit of the work done so far. Running the computations repeatedly (perhaps with different levels of Effort) can lead to improved results. If you have specified Earliest or Latest times for individual artifacts on the Artifacts table, it will probably take longer for the algorithms to find a good answer and you may want to use a smaller Effort value.

Randomize - the heuristic solution technique will normally get the same answer each time you run it, provided you start with the same seriation and parameter settings each time. However, if you check the Randomize box, the heuristic will modify its search somewhat and may find a different answer. Since the heuristic finds a good solution, not necessarily the best, this provides a means for extending a search for a best answer, by doing multiple seriations with Randomize checked. Randomize is a random process and you may not be able to repeat results you get using Randomize. If you want OptiPath to try alternative paths that can be replicated, use the Seed parameter on the Seriations dialog. The difference between Randomize and Shuffle (below) is that Shuffle may result in a worse solution (since it starts the search process all over again from a randomly generated starting point) whereas Randomize will not make things worse (since it starts from your current seriation, only randomizing the process of searching for an improvement).

Shuffle - the heuristic solution technique will normally get the same answer each time you run it, provided you start with the same seriation and same parameter settings each time and have not checked the Randomize box. However, if you start from a different seriation, you may get different results (the heuristic gets a good solution, not necessarily the best). Consequently, you may be able to find a better answer by starting from a different solution. Shuffle allows you to do this. Shuffling will replace the current seriation with a random one (which won't be very good in itself but may allow the heuristic to find a better solution). When you click the Shuffle button a warning dialog will appear to remind you that shuffling will replace your current seriation with a randomly generated one. Click OK to continue. Shuffle, like Randomize, is a random process and you may not be able to repeat the results you get using Shuffle. The difference between Randomize (above) and Shuffle is that Shuffle may result in a worse solution (since it starts the search process all over again from a randomly generated starting point) whereas Randomize will not make things worse (since it starts from your current seriation, only randomizing the process of searching for an improvement).

Compute initiates a new seriation, and then turns into a Stop button. During the computation there will be a progress bar. The heuristic algorithms are iterative. If they make a significant improvement on the solution, they may try again and again until they stop making improvements. You can stop the algorithms with the Stop button without losing the benefit of the work done so far. At any time the dialog shows the value of the starting solution (1.4286 here) and the best solution found so far (0.381) and expresses the improvement as a percentage (73.33%). If you have specified Earliest or Latest times for individual artifacts on the Artifacts window, it will probably take longer for the algorithms to find a good answer.

When the algorithm is finished you will be given the computation time. Without going back to the OptiPath window you can run the algorithms again if you want to see if they can make any further improvements.

Exit is the Exit button will take you back to the main OptiPath window where you can view the results.