Ending child poverty is the entire reason Jacinda Ardern got into politics.

When Ardern announced in Monday's debate that she would march in the streets to end homelessness, it wasn't a political platitude. You could imagine her doing exactly that any day of the week.

Next to Bill English's numbers-driven public persona, Ardern's passion and conviction is perhaps her greatest strength.

But in the nine years that National have been in power, the social policy landscape has shifted, and so too has the public debate and conversation. As Bill English puts it, "Labour does not own compassion and care. They do not own the ability to change lives."

He will be referring to National's own approach to welfare: social investment. Rather than government simply spending money and turning its back, social investment uses data to concentrate spending on areas where it can make the most difference. Or at least, that is what it says on the tin. The contents may differ in practice.

If National want to reclaim compassion, then it is time for Labour to play ball. Labour do not own "compassion and care", but National does not own the better use of data for evidence based policy.

Some aspects of social investment were not even started by National. Ardern pointed out that Labour had been adopting early intervention policies before National came to power. However, the radical part of social investment is the use of data to inform early intervention policies. It is not yet clear whether Labour will keep or discard that part.

Meanwhile some of National's recently announced 'tough on crime' policies do not seem to involve a social investment component at all.

But no matter the definitional issues, call it a social investment approach, call it whatever you like, there are aspects of National's welfare policies that Labour should not just adopt, but improve on.

National's approach to welfare has not been without controversy. There have been accusations that people who go off the benefit are not doing any better than they were. Due to a string of mishaps involving privacy issues, National have lost a lot of trust and goodwill from social services on the frontline. And there is still some scepticism over the extent to which data can really capture lived experiences.

Here is where Labour can do things better.

Measuring outcomes is valuable, but Labour might want to redefine those outcomes. Rather than measure success by how many people are moved off welfare, why not measure and report on what happens when people leave the benefit?

Labour also have an advantage in rebuilding relationships with those working on the ground. Convincing community social services to share the most necessary information can help both them and government understand the difference they are making. But to do that requires trust between government and those serving society's most vulnerable. After all, some of those most in need of help are also those most likely to be wary of government.

Finally, while data on a spreadsheet cannot tell us everything we need to know about delivering social services, it can tell us a lot. Rather than throw the use of data out with the bathwater, that valuable information should be supplemented with qualitative knowledge and experiences.

Both parties have announced ambitious targets for reducing child poverty. But to make a real difference, whatever government we end up with should know and be transparent about whether people's lives have actually changed.

The use of evidence, after all, is not the antithesis of compassion.