Measurement accuracy

The Moodmetric level

The Moodmetric algorithm has been developed for interpreting the electrodermal activity (EDA) signal. It produces an index of 0 to 100 (the so-called Moodmetric or MM level) from the raw signal generated by electrodermal activity.

The MM level is determined from the typical features associated with electrodermal activity: skin reactions per minute (SCR frequency), % of SCL value (SCV value), raw level of skin conductance (SCL).

Accuracy of measurement

Electrodermal activity is a measurable physiological parameter. It reflects the activation of the sympathetic nervous system (SNS) or the fight-or-flight response. The SNS activation impacts directly to sweat gland activity and this change can be measured as variations in skin conductance.

The measurement is done with the help of two electrodes placed on the skin. Changes in skin conductance level are measured in units of microsiemens (us) and in the range of 1-20uS.

The Moodmetric ring measures these conductance changes between two silver coated electrodes with a resolution above the range of detectability threshold of 0.01 uS for modern EDA measurement (A Guide for Analysing Electrodermal Activity (EDA) & Skin Conductance Responses (SCRs) for Psychological Experiments). High temporal resolution in EDA measurement is usually not considered as critical as the conductance level. However, the Nyquist measurement theorem may be used as a good general guideline, where the sample rate should be twice the rate of separable events.

Changes in skin conductance can be divided in two measurable components, slow changing tonic EDA and fast changing non-specific skin response. The latter occurs typically 1-20 times per minute and thus, for long-term measuring 1-5 samples per second is usually sufficient.The Moodmetric EDA measurement uses as default 3 Hz sampling rate to optimize battery life.
The quality of the Moodmetric measurement has been researched in two independent studies. In both, the signal similarity (83%) and the classification power has been found to be closely equivalent to that of laboratory research equipment (Torniainen, 2015) (Pakarinen, 2019).