protected Object currentValue() {
            if (count == 0) {
                return null;
            } else if (count == 1) {
                return 0.0;
            }
            return Math.sqrt(stdDeviation / count);
        }
Standard deviation abstrct aggregator for Double values
modules/siddhi-core/src/main/java/io/siddhi/core/query/selector/attribute/aggregator/StdDevAttributeAggregatorExecutor.java