Release 1.10.1

As the name suggests, the Compare Samples tool tests the capabilities of an HTR model based on a sample rather than a manually selected test set. We have explained in an earlier post how to create such samples, that they represent an objective alternative to conventional test sets and why they can be created with much less effort.

“Compare Samples” may look like a validation tool, but is actually not one of them. You can use it to validate an HTR model, but Advanced Compare is better suited for this.  The real function of “Sample Compare” is to make predictions about the success of an HTR model on a given material.

You may remember the Model Booster. There you need a suitable HTR model that can serve as a base model for a planned HTR training. With the numerous Public Models available, it is a good idea to first check with “Compare Samples” which model fits to your project.

To create such a prediction for a sample, you first have to run the selected HTR models over the entire sample (before that, of course, you have already created the GT for the sample). Then open the Samples tab of the “Compare Samples” tool. This tab lists all samples of your active collection. You select the sample that will be used as the basis for the prediction. Now you can select the model in the middle, whose text version should serve as a reference for the GT. Start “Compute” and you’re done.

The tool now calculates average values for all lines of the sample with an upper bound, a lower bound and an average value. In the range between upper bound and lower bound you should find the Character Error Rate for 95% of your material at which the selected HTR model is expected to work.  In our example below, between 4,7 and 2,9 %.

This way you can compare as many models for your material as you like. But the tool also allows a few other things. For example, you can easily check how an HTR model with or without language model or dictionary works on your material and if it is worth using one or the other. Of course this is especially useful to check your own models.

Tips & Tools
Create several smaller samples rather than one giant sample for all your material. You can separate them chronologically or by writer’s hands, for example. This will allow you to make a differentiated prediction for the use of HTR models on all your material or parts of it.

Release 1.10.1

The Character Error Rate (CER) compares, for a given page, the total number of characters (n), including spaces, to the minimum number of insertions (i), substitutions (s) and deletions (d) of characters that are required to obtain the GT result. If that was not mathematical enough for you:

CER = [ (i + s + d) / n ]*100

This means that even all the little mistakes are statistically full-fledged errors. Every missing comma, a “u” instead of a “v”, an additional space or even an uppercase letter instead of a lowercase letter are included in the CER as “whole errors”. The small details neither disturb the reading and understanding of the text nor do they prevent the search engine from finding a term.
So don’t only look at the numbers but also at the text comparison.Your model is usually better than the CER (and especially the WER) suggest.
To illustrate this, we have calculated this exemplary:

Release 1.10.1

In contrast to the visualization of the errors via the tool “Compare Text Versions” the ordinary “Compare” gives us the same validation results as numerical values.

In addition to the word error rate, we also get the somewhat more conclusive character error rate (CER). Furthermore, in the “Advanced Compare” we can have these results calculated for the whole document or for specific pages in it – always provided that the selected pages have a GT version. Because in Advanced Compare the GT is automatically set as reference.

So select the model to be validated and start the calculation. The result gives you not only the average value for the whole document, but also the corresponding values for each individual page. And that makes the Advanced Compare the most important validation tool in systematic analysis when developing models.

In our rather complex model training for the Spruchakten (over 1000 writer’s hands from more than 150 years) we worked with separate small test sets. On them we could validate our new models over and over again via the Advanced Compare and analyse the results thoroughly. In this way not only average improvements or worsening could be traced in detail. We were also able to identify particular exceptions, such as individual concept fonts or particularly “smeared” ones, which worsened the otherwise good overall result. In addition, we were able to create many graphics from the numerical material, which helped us – and now you – to better understand certain phenomena and developments.

Tips & Tools
You can also download the validation results of the Advanced Compare as an Excel spreadsheet to your computer. To do so, you can select a folder under the result display where you want to save the document. Then click on the button “Download XLS”. Do not just press Enter – otherwise you will have to start all over again.

Release 1.10.1

So, a new HTR model has run over a page and you want to have a first overview on how the model has read? Go to the tool option “Compute Accuracy”, enter the corresponding reference (GT) and hypothesis (HTR Text) and take a look at the validation tool „Compare Text Versions“:

The Text Compare visualizes the comparison of HTR and GT version directly in the text. A word with an error appears red marked and crossed out, behind it you see in green the correct version from the GT. So the text Compare basically shows the word error rate (WER). But above all it allows us to quickly recognize which mistakes exactly were made. So we can also see, for example, that many of the errors are actually minor mistakes, which don’t really bother us when reading and searching for words. In our example here we see a WER of 15%.

Release 1.10.1

There are different ways to measure the accuracy of our HTR-models in Transkribus. Three Compare tools calculate the results and present them in different ways. In all three cases the hypothesis (HTR version) of a text is compared with a corresponding reference (correct version, i.e. GT) of the same text.

The first tool which shows the most immediate result is “Compare Text Versions“. It visualizes the validation for the currently opened page in the text itself. Here we can see exactly which mistakes the HTR has made at which points.

The standard “Compare” gives us these same validation results as numerical values. Among other things, it calculates the average word error rate (WER), the character error rate (CER) and the respective accuracy rates. (If someone knows what the bag tokens are about, he/she is welcome to write us a comment). In the “Compare” we also have the possibility to run the “Advanced Compare“, which allows us to perform the corresponding calculations for the whole document or only for certain pages.

We already have presented the validation tool “Compare Sample” briefly in another post to show how to create Test Samples. The actual Sample Compare then predicts how a model will potentially read on a Test Sample that has been created for this purpose.

Release 1.10.1

We talked about the use of dictionaries in an earlier post and mentioned that the better an HTR model is (CER below 7%), the less useful a dictionary is for the HTR result.
This is different when using Language Models, which are available in Transkribus since December 2019. Like dictionaries, Language Models are generated from the ground truth used in each HTR training. Unlike dictionaries, Language Models do not aim at identifying individual words. Instead, they determine the probability of a word sequence or the frequent combination of words and expressions in a particular context.
Unlike dictionaries, the use of Language Models always leads to much better HTR results. In our tests, the average CER improved by as much as 1% compared to the HTR result without the Language Model – consistently, on all test sets.

Tips & Tools: The Language Model can be selected when configuring the HTR. Unlike dictionaries, Language Models and HTR model cannot be freely combined. Each HTR model has its uniquely genereated Language Model and only this one can be used.

If a project concept does not allow the strategic planning and organization of test sets, there is a simple alternative: automatically generated samples. Samples are also a valuable addition to the manually created test sets, because here it is the machine that decides which material is included in the test and which is not. Transkribus can select individual lines from a set of pages that you have previously provided. So it’s a more or less random selection. This is worthwhile for projects that have a lot of material at their disposal – including ours.

We use samples as a cross-check to our manually created test sets and because samples are comparable to the statistical methods of random sampling, which the DFG also recommends in its rules of practice for testing the quality of OCR.

Because HTR – unlike OCR – is line-based, we modify the DFG’s recommendation somewhat. I will explain this with an example: For our model Spruchakten_M-3K, a check should be carried out on the reading accuracy achieved. For this purpose we created a sample, exclusively from untrained material of the whole period for which the model works (1583-1627). To do this, we selected every 20th page from the entire data set, obtainimg a subset of 600 pages. It represents approximately 3.7% of the total 16,500 pages of material available for this period.  All this is done on your own network drive. After uploading this subset to Transkribus and processing it with the CITlab Advanced LA (16.747 lines were detected), we let Transkribus create a sample from it. It contains 900 randomly selected lines. This is about 5% of the subset. This sample was now provided with GT and used as a test set to check the model.

And this is how it works in practice: The “Sample Compare” function is called up in the “Tools” menu. Select your subset in the collection and add it to the sample set pressing the “add to sample” button. Then specify the number of lines Transkribus should select from the subset. Here you should select at least as many lines as there are pages in the subset, so that each page has one test line.

In our case, we decided to use a factor of 1.5 just to be sure. The program now selects the lines independently and compiles a sample from them, which is saved as a new document. This document does not contain any pages, but only lines. These must now be transcribed as usual to create GT. Afterwards any model can be tested on this test set using the Compare function.