Knowledge Base

I Get Different Results When I Analyze the Same Video Twice

There may be a few reasons for this to happen.

Reason 1 - The effect of the reference image. When you use the detection methods Dynamic Subtraction or Differencing, the image used as reference may differ between trials even when you use exactly the same detection settings. The effect is more pronounced when you use Differencing.

• If you position the video file at different times (thus at different frames) then you go back to the beginning and start the trial, the reference image may be different in the two trials. This will cause the track to differ slightly between the trials.
• If you choose to use the dynamic reference image instead of the saved reference image at the start of each trial in a series (see The Reference Image) then the reference image with which trial 2 starts may not be exactly the same as the one at the start of trial 1. The difference may be of one or few more pixels brighter or darker, but still that difference has an effect.

To ensure that you use a consistent reference image, duplicate the Detection Settings and under Advanced select Background > under Acquisition Settings select Use saved reference image or Use first frame of each trial. This way at the start of each trial the same reference image is used. See The Reference Image.

Reason 2 - Missing samples can affect the track duration. This happens especially if the missing samples happen to occur just when trial control start or stop conditions are evaluated.

Reason 3 - In multi-subject tracking done with Deep Learning, the results are not entirely repeatable due to the non-deterministic nature of the algorithm. Note that this occurs when you select to track two subjects in the arena, not just one. See Troubleshooting: Deep Learning > The Results of Two-Subject Tracking Are Not Consistent When Using the Same Video.

Concluding, to improve the consistency between trials:

• Start both trials at the beginning of the video file, without positioning the video on any frame in the middle.
• Improve the subjects' detection to reduce the number of missing samples.
• Use Gray Scaling or Static Subtraction (if possible). If you use Differencing or Dynamic Subtraction, choose to use a fixed reference image. See The Reference Image.
• To smooth out the tracks and remove outliers, apply the Lowess smoothing to the tracks. See The Lowess Smoothing Method.

The Average Heading Calculated in Excel Does Not Match the Results in EthoVision XT

If you use the ATAN formula, the value is wrong because ATAN uses another convention for the sign of the average sine and cosine of angles. Use instead ATAN2.

The formula for the average heading in a track is:

=DEGREES(ATAN2(AverageCos, AverageSin))

Where AverageCos is the average of Cos values (calculated as =COS(RADIANS(angle))), and AverageSin is the average of Sin values (calculated as =SIN(RADIANS(angle))), and angle is the per-sample value of Heading, Head Direction, or Heading to Point.

There Is No Linear Relationship Between Average Velocity and Total Distance Moved

When plotting the average velocity and the total distance moved, there should be a linear relationship between the two. In particular:

Average Velocity = Total Distance Moved / T

Where T is the time interval (e.g. trial, or time bin) within which the samples are used to do the calculations.

In some cases the data points deviate from the linear relationship. The reason is that the total distance moved is calculated over the available samples. If some samples within an interval are missing, or in some samples the subject was not found, or if the interval is incomplete (for example, the last time bin in a track), then the total distance is underestimated while the average velocity is unaffected. As a result, the mean velocity no longer lies on the straight line; rather, it lies above the line. The mean velocity is higher than expected from the current distance moved. This pattern is therefore an artifact of the variation in the number of samples available for each time interval. In principle, there is nothing wrong with the data, provided that the proportion of missing samples or samples with "subject not found" is low.

I Defined an Active State in the Analysis Profile, However When I Plot the Data I Get Lots of Inactive States

This usually happens when the Activity settings in the Detection Settings are too high. For example, the Activity threshold is 20, or Background noise filter is set to more than 3. In such cases the thresholds are so high that in almost no video frame the subject is considered Active, even if in the Analysis profile the Active state was defined with low threshold values.

Solution: Make a duplicate of your Detection Settings, reduce the Activity threshold and/or the Background noise filter. Check in the video image that purple pixels appear only when the subject moves. Then repeat the tracking with those Detection Settings.

I Get Wrong Statistics of Manually-Scored Behaviors

Problem: I scored a behavior of type "Start-stop". In the Statistics and Charts page, the frequency of Not [behavior name] is X+1 when it should be X.

Explanation: This occurs when you score a Start-stop behavior until the end of the trial. At the last sample time, the behavior is automatically ended, and its complementary Not [behavior name] is scored. This results in the additional occurrence of Not [behavior name] in the statistics. However, the statistics of the actual behavior are correct.

The Results Table Shows Empty Rows or Columns

This occurs often when you make multiple data selections. For example, you make a Nest condition to select the time from 12 hours to 24 hours, then you create time bins of one hour each. The results table also shows the "empty" bins from 0 to 12 hours.

To hide the empty rows/columns:

1. Click the Layout button on the toolbar (or click the Show/Hide button at the top-right corner of the screen and select Layout).
2. In the window that opens, select Hide empty rows or columns.

The Results Table Shows the Same Value for the Same Dependent Variable for the Same Trial, Arena or Subject

This may be due to the fact the results are calculated with a data profile containing two or more Results boxes, but the table does not show the corresponding headers. Click the Layout button and add Selection Result, Arena name, or Subject. See Modify the Layout of the Results Table.

The Results Table Shows Wrong or Missing Units

This occurs when in the Layout window you place the Variable with detail on the rows and the Statistic with unit on the columns, and vice versa. To solve the issue, place both items on the rows or on the columns. See Modify the Layout of the Results Table.

Source: EthoVision XT 19 Help, Noldus Information Technology