Clinton Boys

Australian data scientist and mathematician, living in London.

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The Israeli electoral system is completely different to the Australian system. 120 representatives are elected to the Knesset from a single at-large electorate. Members must be registered to a party; parties submit lists of candidates in an order they want them to be elected, candidates are then elected to the Knesset from these lists based on a proportion of votes received by each party, provided the vote is greater than the electoral threshhold (currently 3.25%, a significant increase from the previous election).

It is difficult to provide a probabilistic model of what the Israeli government will look like after the election, even with the significant amount of polling that exists, because Israel’s proportional representation system has meant, like many European countries in the post-war period, majority governments never occur. Rather, the election is just the first phase of a process that is followed by political wrangling and deals behind closed doors to form a coalition government (and these are often very unstable: the current election is more than twelve months early because of the breakdown of a previous coalition).

In these two posts I just want to come up with a simple poll aggregator that adjusts for accuracy in previous contests. There’s a whole bunch of opinion polls floating around but most aggregators (like the one on HaAretz) just seem to take a simple average (although I imagine there’s plenty of sites in Hebrew that I can’t read yet). For this first step, I want to look at the final poll all the polling firms did before the 2013, 2009 and 2006 elections and come up with an accuracy score for each polling firm which will then translate into a weight in the aggregator.

I looked at the Wikipedia pages for each election, which contain a lot of opinion polling information, and put together lists of final polls before the respective elections (historical_polls) for lots of different polling firms. I also needed the results of each election (results).

import pandas as pd
from pandas import DataFrame

historical_polls = pd.read_csv('israeli_pre_election_polls.csv',delimiter=',')
results = pd.read_csv('israeli_results.csv',delimiter=',')
results = results.set_index('year')
historical_polls = historical_polls.sort('pollster')

The following code produces a fairly simple weighting mechanism, where a pollster with average error receives a weight of 1; below-average pollsters receive less and above-average more.

parties = ['Kadima', 'Likud', 'Yis Bet', 'Labor', 'Shas', 'UTJ', 'Hadash', 'Balad', 'Meretz', 'Yesh', 'Otzma', 'Am', 'Hatnuah', 'NU', 'Greens', 'Gil']

final_frame = pd.DataFrame(columns = historical_polls['pollster'].unique(),index=results.index)

for firm in historical_polls['pollster'].unique():
    sum = 0
    for party in parties:
        sum = sum + abs(results[party] - historical_polls[historical_polls['pollster']==firm].set_index('year')[party])*historical_polls[historical_polls['pollster']==firm].set_index('year')[party]/120
        days = historical_polls[historical_polls['pollster']==firm].set_index('year')['days_before']
    final_frame[firm] = sum

for firm in historical_polls['pollster'].unique():
    means = final_frame.mean(axis=1)
    final_frame[firm] = abs(final_frame[firm] - means)

total_error = pd.DataFrame(final_frame.mean(axis=0))
av_error = total_error.mean()[0]
print av_error
df = 1- (final_frame.mean(axis=0) - av_error)

This produces the following output:

Dahaf               1.412384
Dialog              1.303897
Geocartography      1.352617
Jpost               1.425932
Maagar Mochot       1.137197
Midgam              1.271375
New Wave            0.831736
Panels              1.176537
Shvakim Panorama    0.585856
Smith               0.635624
Sof Hashavua        0.470412
Teleseker           0.396433
dtype: float64
[Finished in 1.2s]

In the next post we’ll incorporate these weights into a poll aggregator.