Knowledge Base

Definition

A variable defined in the Trial Control settings. See Actions > To define a Trial Control variable.

You may want to analyze or visualize the values of a Trial Control variable. For example, you defined a variable named Counter, and:

• You want to calculate the maximum value that the variable reached during different trials.
• You want to visualize its values plotted against time.

How to Specify a Trial Control Variable

1. In the Analysis profile, click the Add button next to Trial Control variable.
2. From the Element list, select the variable you want to analyze.
3. Complete the procedure to add the variable. See Calculate statistics: procedure.
4. To visualize the variable on the timeline, choose the Integrated Visualization.

Notes

• If you want to calculate the time that the Trial Control variable had a specific value, in the Analysis profile choose Multi-condition instead. There you can select the Trial Control variable and define the selection criteria. See Multi condition.
• If you want to calculate the time based on a From - To criterion, choose Free Interval instead. There you can define, for example, an interval from Trial Control variable = 10 to Trial Control Variable = 15. See Free interval.

External Data Variable Definition

EthoVision XT only analyzes external data after resampling. That is, it does not analyze the original data, but a Dependent variable representing the imported signal, resampled to the same sample rate as the EthoVision XT sample rate. This is accomplished by combining upsampling and downsampling (see below).

Downsampling and Upsampling

Downsampling

If the sample rate of the external data signal is higher than the EthoVision XT sample rate, the signal is downsampled. For each sample k, a new value is calculated from the values of the original signal in the interval (k-1; k] (or in multiple preceding sample intervals, depending on the Averaging interval chosen). The interval is half-closed, that is, all values of the signal between k-1 (not included) and k are considered. The new value gets the time stamp of sample k.

Five methods are available to downsample the signal:

• Last value
• Maximum
• Mean
• Minimum
• Total value

Upsampling

If the sample rate of the external data signal is lower than the EthoVision XT sample rate, the signal is upsampled. For each sample k, a new value is calculated from the values present in the sample interval (k-1; k] (or in multiple preceding sample intervals, depending on the Averaging interval chosen). If no value is found, a new value is interpolated using one of the methods available, or replaced by missing sample or a zero value.

Four upsampling methods are available:

• Last value
• Linear interpolation
• Missing value
• Zero value

Averaging Interval

With Averaging interval you can smooth the values of the converted signal. This has an effect on both resampled and state variables.

• With Averaging interval 1, the new value for sample k only depends on the original values of the signal in the interval (k-1, k].

  Example: Downsampling an EEG signal with the Mean value method and an Averaging interval of 1.

• With Averaging interval x (where x = 2, 3, ...), the new value for sample k depends on the original values of the signal in the interval (k-x, k].

  Example: Downsampling an EEG signal with the Mean value method and an Averaging interval of 3. The signal is downsampled by calculating averages over 3 sample intervals, then averaging those three averages.

Special Case: Resampling

If the sample rate of the external data signal is the same as the EthoVision XT sample rate, but with different time stamps, the signal is re-sampled to align its values to the EthoVision XT samples. Values are shifted to the right.

How to Specify an External Data Variable (Resampled)

1. Click the Add button next to the [data set name].
2. Under Select, choose the Downsampling method (default: Mean value) and the Upsampling method (default: Last value).
3. Under Outlier filter, select the Averaging interval (default: 1).
4. Complete the procedure to add the variable. See Calculate statistics: procedure.
5. Plot Integrated Data or calculate the statistics.

Notes

• If you upsample a signal with a sample rate much lower than the EthoVision sample rate, and you choose Missing value as the Upsampling method, it may be difficult to see the data points. In Integrated Visualization, choose Show/Hide > Show Graph Data Points and zoom in the plot.
• See also Averaging interval.

Source: EthoVision XT 19 Help, Noldus Information Technology