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Readability Index in Python(NLP)

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Readability is the ease with which a reader can understand a written text. In natural language, the readability of text depends on its content (the complexity of its vocabulary and syntax). It focuses on the words we choose, and how we put them into sentences and paragraphs for the readers to comprehend. 

Our main objective in writing is to pass along information that both the writer and the reader think is worthwhile. If we fail to convey that information, our efforts are wasted. In order to engage the reader, it’s critical to present information to them that they’ll gladly keep reading and be able to understand clearly. So, it is required that the content be easy enough to read and understand this is as readable as possible. There are various available Difficulty Scales with their own difficulty determining formulae.

This article illustrates various traditional readability formulae available for readability score evaluation. In Natural Language Processing, sometimes it is required to analyze words and sentences to determine the difficulty of the text. Readability Scores are generally grade levels on particular scales, which rates the text as to whats the difficulty of that particular text. It assists the writer in improving the text to make it understandable for a larger audience, thus making content engaging. 

Various available Readability Score Determination Methods/Formulae:

  1. The Dale–Chall formula
  2. The Gunning fog formula
  3. Fry readability graph
  4. McLaughlin’s SMOG formula
  5. The FORECAST formula
  6. Readability and newspaper readership
  7. Flesch Scores

Read about more available Readability Formulae from here.

The implementation of the readability formulae is shown below. 

The Dale Chall Formula:

To apply the formula:
Select several 100-word samples throughout the text. 
Compute the average sentence length in words (divide the number of words by the number of sentences). 
Compute the percentage of words NOT on the Dale–Chall word list of 3, 000 easy words. 
Compute this equation 

 Raw score = 0.1579*(PDW) + 0.0496*(ASL) + 3.6365
PDW = Percentage of difficult words not on the Dale–Chall word list.
ASL = Average sentence length

The Gunning fog Formula

Grade level= 0.4 * ( (average sentence length) + (percentage of Hard Words) )
Here, Hard Words = words with more than two syllables.

Smog Formula

SMOG grading = 3 + √(polysyllable count).
Here, polysyllable count = number of words of more than two syllables in a 
sample of 30 sentences.

Flesch Formula

Reading Ease score = 206.835 - (1.015 × ASL) - (84.6 × ASW)
ASL = average sentence length (number of words divided by number of sentences)
ASW = average word length in syllables (number of syllables divided by number of words)

Advantages of Readability Formulae:
1. Readability formulas measure the grade-level readers must have to be to read a given text. Thus provides the writer of the text with much-needed information to reach his target audience. 
2. Know beforehand if the target audience can understand your content.
3. Easy-to-use. 
4. A readable text attracts more audience. 

Disadvantages of Readability Formulae:
1. Due to many readability formulas, there is an increasing chance of getting wide variations in results of a same text. 
2. Applies Mathematics to Literature which isn’t always a good idea.
3. Cannot measure the complexity of a word or phrase to pinpoint where you need to correct it. 


import spacy
from textstat.textstat import textstatistics,legacy_round
# Splits the text into sentences, using
# Spacy's sentence segmentation which can
def break_sentences(text):
    nlp = spacy.load('en_core_web_sm')
    doc = nlp(text)
    return list(doc.sents)
# Returns Number of Words in the text
def word_count(text):
    sentences = break_sentences(text)
    words = 0
    for sentence in sentences:
        words += len([token for token in sentence])
    return words
# Returns the number of sentences in the text
def sentence_count(text):
    sentences = break_sentences(text)
    return len(sentences)
# Returns average sentence length
def avg_sentence_length(text):
    words = word_count(text)
    sentences = sentence_count(text)
    average_sentence_length = float(words / sentences)
    return average_sentence_length
# Textstat is a python package, to calculate statistics from
# text to determine readability,
# complexity and grade level of a particular corpus.
# Package can be found at
def syllables_count(word):
    return textstatistics().syllable_count(word)
# Returns the average number of syllables per
# word in the text
def avg_syllables_per_word(text):
    syllable = syllables_count(text)
    words = word_count(text)
    ASPW = float(syllable) / float(words)
    return legacy_round(ASPW, 1)
# Return total Difficult Words in a text
def difficult_words(text):
    nlp = spacy.load('en_core_web_sm')
    doc = nlp(text)
    # Find all words in the text
    words = []
    sentences = break_sentences(text)
    for sentence in sentences:
        words += [str(token) for token in sentence]
    # difficult words are those with syllables >= 2
    # easy_word_set is provide by Textstat as
    # a list of common words
    diff_words_set = set()
    for word in words:
        syllable_count = syllables_count(word)
        if word not in nlp.Defaults.stop_words and syllable_count >= 2:
    return len(diff_words_set)
# A word is polysyllablic if it has more than 3 syllables
# this functions returns the number of all such words
# present in the text
def poly_syllable_count(text):
    count = 0
    words = []
    sentences = break_sentences(text)
    for sentence in sentences:
        words += [token for token in sentence]
    for word in words:
        syllable_count = syllables_count(word)
        if syllable_count >= 3:
            count += 1
    return count
def flesch_reading_ease(text):
        Implements Flesch Formula:
        Reading Ease score = 206.835 - (1.015 × ASL) - (84.6 × ASW)
          ASL = average sentence length (number of words
                divided by number of sentences)
          ASW = average word length in syllables (number of syllables
                divided by number of words)
    FRE = 206.835 - float(1.015 * avg_sentence_length(text)) -\
          float(84.6 * avg_syllables_per_word(text))
    return legacy_round(FRE, 2)
def gunning_fog(text):
    per_diff_words = (difficult_words(text) / word_count(text) * 100) + 5
    grade = 0.4 * (avg_sentence_length(text) + per_diff_words)
    return grade
def smog_index(text):
        Implements SMOG Formula / Grading
        SMOG grading = 3 + ?polysyllable count.
           polysyllable count = number of words of more
          than two syllables in a sample of 30 sentences.
    if sentence_count(text) >= 3:
        poly_syllab = poly_syllable_count(text)
        SMOG = (1.043 * (30*(poly_syllab / sentence_count(text)))**0.5) \
                + 3.1291
        return legacy_round(SMOG, 1)
        return 0
def dale_chall_readability_score(text):
        Implements Dale Challe Formula:
        Raw score = 0.1579*(PDW) + 0.0496*(ASL) + 3.6365
            PDW = Percentage of difficult words.
            ASL = Average sentence length
    words = word_count(text)
    # Number of words not termed as difficult words
    count = word_count - difficult_words(text)
    if words > 0:
        # Percentage of words not on difficult word list
        per = float(count) / float(words) * 100
    # diff_words stores percentage of difficult words
    diff_words = 100 - per
    raw_score = (0.1579 * diff_words) + \
                (0.0496 * avg_sentence_length(text))
    # If Percentage of Difficult Words is greater than 5 %, then;
    # Adjusted Score = Raw Score + 3.6365,
    # otherwise Adjusted Score = Raw Score
    if diff_words > 5:      
        raw_score += 3.6365
    return legacy_round(score, 2)


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Last Updated : 03 Sep, 2021
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