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The use of numerical estimates of base calling accuracy

In this section we give an overview of our use, when available, of base call accuracy estimates or confidence values. We also explain the importance of the consensus calculations used by gap4, and their role in minimising the work needed to complete sequencing projects.

We first put forward the idea of using numerical estimates of base calling accuracy in our paper describing SCF format Dear, S. and Staden, R, 1992. A standard file format for data from DNA sequencing instruments. DNA Sequence 3, 107-110 and then expanded on their use for editing and assembly in Bonfield,J.K. and Staden,R. The application of numerical estimates of base calling accuracy to DNA sequencing projects. Nucleic Acids Res. 23, 1406-1410 (1995).

In Bonfield and Staden (1995), we stated "...the most useful outcome of having a sequence reading determined by a computer-controlled instrument would be that each base was assigned a numerical estimate of its probability of having been called correctly... having numerical estimates of base accuracy is the key to further automation of data handling for sequencing projects. ... The simple procedure we propose in this paper is a method of using the numerical estimates of base calling accuracy to obviate much of the tedious and time consuming trace checking currently performed during a sequencing project. In summary we propose that the numerical estimates of base accuracy should be used by software to decide if conflicts between readings require human expertise to help adjudicate. We argue that if the accuracy estimates are reasonably reliable then the majority of conflicts can be ignored... and so the time taken to check and edit a contig will be greatly reduced."

This has been achieved by making the consensus calculations (see section The Consensus Calculation) central to gap4, and by providing calculations which make use of base call accuracy estimates to give each consensus base a quality measure. The consensus is not stored in the gap4 database but is calculated when required by each function that needs it, and hence always takes into account the current data. In the Contig Editor the consensus is updated instantly to reflect any change made by the user.

In 1998 the first useable probability values became available through the program Phred (Ewing, B. and Green, P. Base-Calling of Automated Sequencer Traces Using Phred. II. Error Probabilities. Genome Research. Vol 8 no 3. 186-194 (1998)). Phred produces a confidence value that defines the probability that the base call is correct. This was an important step forward and these values are widely used and have defined a decibel type scale for base call confidence values. Gap4 is currently set to use confidence values defined on this scale.

The confidence value is given by the formula

     C_value = -10*log10(probability of error)

A confidence value of 10 corresponds to an error rate of 1/10; 20 to 1/100; 30 to 1/1000; and so on. Using the main gap4 consensus algorithm they enable the production of a consensus sequence for which the expected error rate for each base is known.

As is described elsewhere (see section List Consensus Confidence) being able to calculate the confidence for each base in the consensus sequence makes it possible to estimate the number of errors it contains, and hence the number of errors that will be removed if particular bases are checked and, if necessary, edited. For example, if 1000 bases in the consensus had confidence 20, we would expect those 1000 bases (with an error rate of 1/100) to contain 10 errors.

Another program which produces decibel scale confidence values for ABI 377 data is ATQA Daniel H. Wagner, Associates, at

For gap4 the confidence values are expected to lie in the range 1 to 99, with 0 and 100 having special meanings to the program.

The confidence values are stored in SCF or Experiment files and copied into gap4 databases during assembly or data entry.

The searches provided by the Contig Editor (see section Searching) are one of gap4's most important time saving features. The user selects a search type, for example to find places where the confidence for the consensus falls below a given threshold, and the search automatically moves the cursor to the next such position in the consensus. The Contig Editor locates the next problem by applying the consensus calculation to the contig. To edit a contig the user selects "Search" repeatedly, knowing that it will only move to places where there is a conflict between good data or where the data is poor. Note that the program is usually configured to automatically display the relevant traces for each position located by the search option.

The main result is that far fewer disagreements between data are brought to the attention of the user and fewer traces have to be inspected by eye, and so the whole process is faster. Another consequence of the strategy is that, as fewer bases need changing to produce the correct consensus, most of what appears on the screen will be the original base calls. Indeed we have taken this a step further and suggest that if a base needs changing because it has a high accuracy estimate, and is conflicting with other good data, then rather than change the character shown on the screen, the user should lower its accuracy value. By so doing more of the original base calls are left unchanged and hence are visible to the user. There is a function within the contig editor to reset the accuracy value for the current base to 0. Alternatively the accuracy value for the base that is thought to be correct can be set within the contig editor to 100.

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