Western Electric Rules SPC Python Script Generator for Fab Data

import csv

CSV_PATH = 'thickness_measurements.csv'
VALUE_COL = 'oxide_thickness_nm'
CENTER = 100.0
SIGMA = 1.2
PARAM = 'Oxide thickness (nm)'


def load_values(path, col):
    values = []
    with open(path, newline='') as handle:
        for row in csv.DictReader(handle):
            values.append(float(row[col]))
    return values


def western_electric(values, center, sigma):
    z = [(v - center) / sigma for v in values]
    flags = []
    for i in range(len(z)):
        if abs(z[i]) > 3:
            flags.append((i, 'Rule 1: point beyond 3 sigma'))
        if i >= 2:
            window = z[i - 2:i + 1]
            for sign in (1, -1):
                if sign * z[i] > 2 and sum(1 for a in window if sign * a > 2) >= 2:
                    flags.append((i, 'Rule 2: 2 of 3 beyond 2 sigma, same side'))
        if i >= 4:
            window = z[i - 4:i + 1]
            for sign in (1, -1):
                if sign * z[i] > 1 and sum(1 for a in window if sign * a > 1) >= 4:
                    flags.append((i, 'Rule 3: 4 of 5 beyond 1 sigma, same side'))
        if i >= 7:
            window = z[i - 7:i + 1]
            if all(a > 0 for a in window) or all(a < 0 for a in window):
                flags.append((i, 'Rule 4: 8 in a row on one side of center'))
    return flags


if __name__ == '__main__':
    data = load_values(CSV_PATH, VALUE_COL)
    print('Parameter:', PARAM, '| points:', len(data))
    results = western_electric(data, CENTER, SIGMA)
    if not results:
        print('No rule violations found.')
    for index, message in results:
        print('Point', index + 1, 'value', data[index], '-', message)