In my field (pharma industry) efficacy is about a medicine that works in ideal controlled conditions (e.g. a clinical study) while effectiveness means that the medicine works in real world conditions, which are usual not controlled and far from ideal.
Report Ana's answer
Both are forms of measurement.
EFFICIENCY - The measurement of TIME AND RESOURCES consumed for a given process to achieve an end product or service.
EFFECTIVENESS - The measurement of end product QUALITY AND PERFORMANCE against a requirements specification for a product or service.
Report Kenneth's answer
Efficient is a word that s root expresses a meaning but also, a type of slang. When the word is used it often has a double meaning. At least in some industries. Efficiency on the other hand is a more direct expression of value with ends facts to determine efficiency.