The Moodmetric level
The Moodmetric algorithm was 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).
The basic principle of the Moodmetric algorithm is that it compares changes in electrodermal activity to the typical, standard values measured for the particular individual over the time of wearing the Moodmetric ring. The values are counted since the last calibration.
Each calibration means a reset, kicking off the collecting of new measurement values from the beginning. This means that only after several hours of using the ring there are enough measurement points to give an accurate MM level value for the wearer of the ring.
The calibration should be done when the ring is first taken into use, or when the user changes. (This is done simply by tapping ‘Calibrate’ at the app Ring info view). Calibration should only be done once for one user. The recommended calibration time is about 12 hours.
Moodmetric measurements can be done without calibration time if certain inaccuracy is allowed.
The calibration can be enhanced by helping the user to both relax and get excited/agitated before the actual measurement. This can be done just prior to the measurement in cases where the user can not wear the ring starting from the previous day.
Note that the calibration time is not needed when only the raw signal is examined. The raw signal can be viewed at the app (the violet curve), streamed to a PC with a dongle or used by other mobile apps benefiting the Moodmetric SDK’s Instant Indicator.