Eric Pickles’s comments  about Romanian and Bulgarian immigration, followed by today’s intervention by Migration Watch  on the same issue, have sparked another lively media debate about migration policy.
The government have based their migration policy on a target to reduce net migration to less than 100,000 a year, one they seem likely to miss , not least because they cannot limit migration from elsewhere in the EU. Ed Miliband  has suggested that Labour’s approach to migration will go beyond the confines of immigration rules (as it must, in light of EU migration). His interventions in the debate have focused on how economic and integration policy can respond to migration.
But underneath the regular political and policy arguments lies a more fundamental debate about what migration policy should be seeking to achieve. The Conservatives’ answer is that the purpose of migration policy is to reduce net migration. That is appealingly simple, but bad policy and bad politics .So what might a centre-left alternative  look like?
The first thing to say is that the most fundamental responsibility of governments of all political hues should be to ensure that migration policy is democratically accountable, governed by the rule of law (including human rights law), and effectively implemented. Of course, this is easier said than done, and is sometimes uncomfortable for those on the left, involving as it does both difficult conversations with the public and highly sensitive decisions about individual cases involving vulnerable people.
Beyond this very basic responsibility, there is plenty of room for debate about what governments should try to achieve with migration policy. Many people would agree that one important objective should be to secure economic benefits for the UK. Indeed, this was arguably the driving motivation behind much of Labour’s immigration policy when in government, and has been a key criticism  of the coalition’s approach.
But a centre-left or progressive migration policy must aim to do more than simply respond to the needs of business. Migration policy must be part of a strategy to form the economy we want, not just a tool to service the needs of the one that we have. A progressive migration policy must also seek to manage the cultural and social impacts of migration, take particular care to avoid increasing inequality, and pay particular attention to the impacts on vulnerable groups. This means tackling migration’s impacts not only through immigration rules, but also through wider social policy. Labour did not do enough of this in government, particularly in response to rapid migration from Poland and other countries after 2004.
However, there is a risk that this kind of approach to migration policy loses touch with the core metric that concerns the public: the impact of policy on migration flows. This was the trap that Labour fell into after 2004, when dramatic increases in immigration exposed a policy approach that seemed to the public to be much to laissez-faire on the issue of numbers.
The Conservatives' net migration target is wrong (not least because it over-simplifies and ignores much about migration flows that is important), but that does not mean that anyone in the migration debate can ignore the question of numbers. It is not enough to set immigration rules and then take no view on the pace and pattern of migration flows that result. Even those who believe that migration has generally been a good thing for the UK should accept that this does not mean that more migration would necessarily be even better: both costs and benefits are non-linear.
People on all sides of the immigration debate should agree that accurate and timely data on the scale, nature and pattern of migration flows are essential for good policymaking. This data is something the UK currently lacks, which makes the current debate about net migration levels even less productive. It is also reasonable to expect politicians and policymakers to respond quickly to rapid changes in migration. The right response will not always be about the immigration rules (indeed, as with EU migration, there are many circumstances when it cannot be), but the public are right to expect government to take steps to manage the impacts of migration.
The right approach to migration policy is necessarily more complicated then the Conservative’s net migration target. But it is also common sense: have a democratic debate, set fair rules and enforce them, increase the benefits (economic and social) of migration while managing the costs, pay attention to impacts on the most vulnerable, and keep a weather eye on the numbers.
The devil is in the detail, of course, but perhaps if everyone in the migration debate could agree on the question, we would get better answers.
Sarah Mulley is associate director at IPPR 
She tweets as @sarahmulley