Less than a quarter of companies earned high marks for disclosing anti-trafficking and forced labor corporate practices in a pilot study released on Thursday to highlight efforts by companies to be open about their supply chains.

The study by KnowTheChain, founded by San Francisco-based Humanity United, found only four of 20 companies were fully open about disclosure and the way they track and deal with forced labor in their supply chains.

The number of people globally living as slaves, trafficked into brothels or forced labor, is estimated at nearly 36 million by the Australia-based Walk Free Foundation with the slavery business estimated to be worth $150 billion a year.

Anti-slavery campaigners have called for companies to be more transparent over their supply chains and for consumers to insist on knowing where goods come from and who makes them.

KnowTheChain said it plans to publish benchmark reports this year comparing corporate policies and practices on keeping forced labor out of supply chains and the pilot study was conducted to test its methodology.

"Overall, the research finds a wide distribution in companies' transparency on efforts to mitigate human trafficking and forced labor in their supply chains," the report said.

"We are doing this with the belief that leaders should be recognized, laggards should be incentivized to improve, and all companies should have insights into how their peers are addressing these challenges so they can strive to do better."

The pilot study found most companies, 17 out of 20, have formal policies to address the potential for human trafficking and forced labor, but none disclosed management incentives linked to addressing those issues.

Few companies demonstrated that their policies are available to vulnerable parties in their supply chains such as local workers, and few translate key documents into local languages to make them accessible.

Also, few of the companies disclosed the names and locations of their first-tier suppliers, the report said.

All the companies have auditing processes to measure suppliers' compliance with their supply-chain standards, but only three in 20 conduct interviews with sub-contracted workers at their supplier locations as part of this process, it said.

Apparel and footwear companies performed better than those in food and beverages and those in information and communication technology, it said.

The study did not disclose individual performances of the 20 companies which included Gap Inc, Nike Inc, Nestle S.A., Unilever plc, Apple Inc, Hennes & Mauritz AB and Samsung Electronics Co Ltd.

The companies were selected based on a sector's high risk of exposure to the issue and a company's market capitalization.