This is the thirty-eighth in a series of posts by Dr Robert Ford, Dr Will Jennings, Dr Mark Pickup and Prof Christopher Wlezien that report on the state of the parties in the UK as measured by opinion polls. By pooling together all the available polling evidence, the impact of the random variation that each individual survey inevitably produces can be reduced.

Most of the short term advances and setbacks in party polling fortunes are nothing more than random noise; the underlying trends – in which the authors are interested and which best assess the parties’ standings – are relatively stable and little influenced by day-to-day events. If there can ever be a definitive assessment of the parties’ standings, this is it. Further details of the method we use to build our estimates of public opinion can be found here.

This was a month of bouncing polls. Early on in June, several polls pointed to an unexpected rebound in Labour’s fortunes, leading to a brief flurry of speculation about a Labour surge. Then, right at the end of the month (and mostly outside of the window that our latest estimates refer to) polls started to show a slight recovery for the Conservatives, which was immediately labelleda “Juncker bounce” by the media, particularly the parts of it who approved of David Cameron’s fruitless campaign to prevent former Luxemboug Prime Minister Jean-Claude Juncker from taking over as President of the European Commission.

The Polling Observatory’s method tends to have a more conservative view of moves in public opinion. Our estimates for the first of July put Labour at 34.6%, up 0.8 points on a month ago. While this is a modest rebound, it nonetheless represents a reversal of the downward trend evident for most of 2014 to date, and is the first significant up-tick in support for Labour since the autumn of last year. Conservative support is stable at 30.8%, down just 0.1 points on last month. However, this is without most of the alleged “Juncker bounce” polls collected in the first week of July, which, when added in, may push the Conservatives modestly higher than they were in late May – but this remains to be seen.

There is little evidence yet of a fall in UKIP support now the European Parliament elections have passed, confounding the expectations of pundits who believed the European election victory was the “peak UKIP moment”. Our estimates have Farage’s party at 14.8%, down just 0.1% on last month. The Liberal Democrats, however, continue to slide to new record lows. This month they register just 8.8%, down 0.5% on last month, and an all-time low under our new methodology.

While our model does register significant month-on-month, and even week-on-week, shifts in public opinion, these are never as dramatic as those shown in the polls which grab the most headlines. The truth is that such bounces are far too large to be plausible as real movements in public opinion – a 7 point swing, for example, would require 2 million people to change their vote preferences in a single week or month. This simply does not happen in the absence of a very powerful change in the political context. It is just not very plausible to believe that 2 million people switched to Labour at the beginning of the month, without any compelling reason to do so, or that a similar mass of voters were won over to the Conservatives by Cameron’s quixotic anti-Juncker campaign.

Once the polls are aggregated together, and the noise inevitably produced by random sampling variation is filtered out, the bounces in public opinion from month to month become much smaller. The largest weekly shifts in support we find to date in this Parliament mostly occur at the very beginning, when Lib Dem support fell by 1.5% in the second week after the general election, and then carried on falling at a similar rate for several weeks afterwards, while Labour support shifted upwards at a similar rate.

In fact, almost half of the 20 largest weekly changes in public opinion in this Parliament are accounted for by the Lib Dems’ post-election collapse. This shift in preferences followed a hugely significant and largely unexpected event – the formation of a coalition between the Liberal Democrats and the Conservatives. Millions of Liberal Democrats who had regarded the party as an ideological stable mate of Labour saw their vote choices in a new light and changed their preferences accordingly.

The other major shift in voters’ preferences during this parliament (aside from a sharp, but short lived, rally in Conservative support immediately after David Cameron’s European summit veto in December 2011) came in the aftermath of the “omnishambles” budget of March 2012, with the Conservatives’ poll rating falling around a percentage point three weeks in a row, and.

Even then, there are several different explanations for this dramatic shift in preferences (as we discussed at the time here), which may have combined to make something of a perfect political storm – wrecking the Conservatives’ reputation for competence, alienating previously supporting groups, and reinforcing negative stereotypes about the ‘nasty party’.

Events such as the formation of a governing coalition between two parties that were not regarded as natural allies can produce large swings in the polls, so can highly visible examples of incompetence or economic crises. However, the vast majority of political events are nowhere near as significant.

This is why the correct initial reaction to any headline of the form “x produces bounce in polls” is “let’s wait and see”. In the vast majority of cases, the apparent realignment of voters is swiftly revealed to be a statistical phantom.