The much-trumpeted "by-election" effect failed to work its magic for the opposition this time round.

After losing two previous by-elections, the People's Action Party (PAP) won Bukit Batok with 61.2 per cent of the vote on May 7.

Observers believe that in a by-election, voters are more willing to support the opposition as there is no danger of inadvertently voting the Government out of power.

So why did the by-election effect not propel the Singapore Democratic Party (SDP) to victory?

First, for all its purported potency, it is not easy to pinpoint specific instances of the by-election effect at work.

In the first 15 years of independence, the PAP handily won all 11 by-elections that were contested in a bygone era when the ruling party held every single seat in Parliament.

Then came the seminal 1981 Anson by-election, which saw the PAP vote share plummet from 84.1 per cent to 47.1 per cent within a year - a 37 percentage point swing.

While this is a seemingly powerful example of the by-election effect, the PAP's analysis identified a unique local issue that was a key reason for the defeat.

A large chunk of the voters might have expressed their dissatisfaction at the ballot box as they were going to be resettled and were not given priority in getting new Housing Board flats.

Since Anson, there have been four by-elections, and the Workers' Party (WP) has won two of them.

But those two victories also do not conclusively prove this by-election phenomenon.

The 2012 Hougang by-election was contested in the WP stronghold, which it had been holding since 1991. It came as no surprise that it easily retained the seat. In the 2013 Punggol East by-election, the WP won a much closer fight.

But there were also unresolved local issues such as uncompleted upgrading works at Rivervale Plaza and a lack of coffee shops in the new estate.

The PAP's candidate was a new face that was unfamiliar, while the WP candidate had a degree of recognition, having contested there in the previous general election.

To find the most convincing instance of the by-election effect, one has to ironically look at the 1991 General Election.

Opposition MP Chiam See Tong devised a strategy to contest fewer than half the parliamentary seats so that the PAP was returned to power immediately on Nomination Day. With the guarantee of a PAP Government, voters were thus free to vote for the opposition, he argued.

Fighting a general election as a by-election would achieve what he termed a "by-election effect", a turn of phrase that endures in Singapore politics till today.

The strategy is widely credited with delivering four elected parliamentary seats to the opposition, a record that stood until 2011.

But is the by-election effect still relevant in today's political climate, where voters have shown an increasing desire for diversity in Parliament and more alternative voices?

After all, the biggest opposition breakthrough to date - the WP's win in Aljunied GRC, the first time the opposition won a GRC - was achieved in the 2011 General Election when all but one constituency were contested.

The opposition has also not pursued the by-election strategy in the last three general elections, contesting more than half the seats in 2006, 2011 and 2015.

Last year's elections even saw all constituencies challenged for the first time since independence.

The result of the Bukit Batok by-election seems to confirm the receding influence of the by-election effect. Undoubtedly, some voters felt unencumbered to vote for SDP's Dr Chee Soon Juan because they were merely choosing an MP and not determining the government of the day.

But perhaps Bukit Batok merely confirms what past results have suggested: The by-election effect does not exist in isolation, as other factors such as local issues and the quality and familiarity of the candidate play a crucial role too.