public function smogIndex($strText = false)
    {
        $strText = $this->setText($strText);

        $score = Maths::bcCalc(
            1.043,
            '*',
            Maths::bcCalc(
                Maths::bcCalc(
                    Maths::bcCalc(
                        Syllables::wordsWithThreeSyllables($strText, true, $this->strEncoding),
                        '*',
                        Maths::bcCalc(
                            30,
                            '/',
                            Text::sentenceCount($strText, $this->strEncoding)
                        )
                    ),
                    'sqrt',
                    0
                ),
                '+',
                3.1291
            )
        );

        if ($this->normalise) {
            return Maths::normaliseScore($score, 0, $this->maxGradeLevel, $this->dps);
        } else {
            return Maths::bcCalc($score, '+', 0, true, $this->dps);
        }
    }