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Staircase Method Element

Staircase Method Element allows an easy implementation of the weighted up/down staircase method with the fixed predefined performance rate (Kaernbach, 1991). The element collects various statistics and provides a dynamic staircase plot, which can be monitored on the status screen.

Table of Contents [Hide/Show] Summary Properties Description Snapshots Practical Use     Creating a staircase Notes Demos

Description ¶

Staircase element assists in implementing the single staircase method that efficiently estimates participant's detection threshold in regards to stimulus intensity (e.g. contrast or duration). The element continuously calculates adjustments for stimulus intensity on the basis of participant's consecutive responses. In addition, the element estimates the threshold of the given participant, collects various statistics and even generates the real-time staircase plot for the status screen. In general, element's staircase algorithm resembles the classical staircase procedure introduced by von Békésy (1947). More particularly, the current algorithm is a replica of the weighted up/down staircase method with the fixed performance rate (Kaernbach, 1991). The element implements a single staircase. If multiple staircases, interleaved or not, are required by an experimental paradigm, several Staircase elements can be used in parallel. As it is shown below, only a new lines of user code are required to make use of Staircase Method element in any experiment.

Snapshots ¶

Example of the staircase plot on Status Screen

Practical Use ¶

Creating a staircase ¶

Design time

1. Add Staircase element to any of experiment's event. The event choice is not important for elements, like Staircase Method, working in the global scope of the experiment.
2. Set a value of Initial Intensity property of the element. The numerical value should correspond the initial level of stimulus intensity that can be easily detected. For example, the number can be related to the opacity of a picture, or to duration of an audio.
3. Set a value of Staircase Step property. The signed value denotes an adjustment of the stimulus intensity when the correct response is given by a participant. The adjustment on the wrong response is calculated by the element automatically as suggested by the weighted method. Smaller step value may lead to more accurate but slower staircase convergence.
4. Set a percentage value of Performance Level property. The staircase procedure will be automatically adapted to keep participant's performance on the given fixed level. Thus, the staircase results will always show participant's detection threshold on the given performance level, e.g for 80% of detection rate. The chance level corresponds to the value 50.
5. Set a value of Reversal Number property to define a total number of reversals necessary to complete the staircase procedure. Reaching the maximal reversal number does not stop the staircase procedure, the element simply signals this moment by setting true to its Is Completed property, which, in turn, can be used to control the experiment flow logic.
6. Set a value of Ignorable Reversal greater than zero, if it's necessary to exclude several initial reversals from the threshold estimation, e.g. to avoid occasional outliers before convergence of the staircase close to the real threshold level.
7. Create proxy variables for AddResponse and Next Intensity properties of the element. The first proxy will be used to add participant's responses to the staircase whereas the second for the adjusting stimulus intensity.
8. Create proxy variables for the selected properties of Runtime Results category. The proxies will allow to monitor staircase statistics on the Status Screen and save results to Report.
   Runtime
9. In order to add a single participant response to the staircase, the trial outcome has to be added via AddResponse proxy (usually, at the end of a trial):
   

   
   
   if (Response==1)
   
   AddResponse=true;
   
   else
   
   AddResponse=false;
   
   

   
   Integer values can be assigned to AddReponse proxy as well:
   

   
   
   AddResponse=Response; /// the Response here is an integer variable, which is equal 1 or 0 depending on the trial outcome
   
   

10. Adjust the intensity of the stimulus with Next Intensity value. A new value is calculated each time after adding a participant response (via AddResponse). At the start of the staircase Next Intensity is equal to Initial Intensity
    

    
    
    RendererOpacity=NextIntensity; /// adjusts the opacity of the visual stimulus
    
    

    
    or,
    

    
    
    EventDuration=Convert.ToUInt64(NextIntensity); /// adjusts duration of the stimulus presentation
    
    

11. After completion of the current staircase, it can be reset and used again in a new sequence of trials. Use the following code:
    

    
    
    ResetStaircase=true; /// ResetStaircase is a proxy variable linked to element's property of the same name.
    
    

12. The overall staircase progress (changes on the stimulus intensity over time) can be written to Report with the following code:
    

    
    
    Report="Stairscase of the stimulus intensity:\r\n";
    
    for (int i=0;i<StaircasePoints.Length;i++)
    
    Report=Report+StaircasePoints[i]+";"; /// where StaircasePoint is a proxy variable linked to element's property of the same name.
    
    

13. The graphical representation of the staircase progress can be monitored on the status screen with Staircase Plot property. The following code for Status Screen shows XAML binding to a proxy linked with the Staircase Plot property:
    

    
    
    <Image Source="{Binding StaircasePlot}"/>
    
    

    
    

Notes ¶

Adding participant's response to the staircase and adjusting the stimulus intensity with the next value MUST NOT be done in the same code snippet (the element estimates a new intensity after completion of the snippet where a new response is added). Instead, use separate consequent snippets in the experiment flow for these two actions. The example of the incorrect code is below:

/// Incorrect use of the staircase !!!
AddResponse=Response;
EventDuration=Convert.ToUInt64(NextIntensity); /// Next Intensity is not updated yet

Demos ¶

Demo experiment that shows an use of the StaircaseMethod can be found in: \Demos\Elements\Psychophisical_Methods\Staircase_Element_Demo.eve

	Name	Size
	Staircase-Method-Element-Icon.png	4.00 KB
	StaircasePlot.png	44.37 KB
	StaircasePlotsm.png	28.74 KB